Automatic Load Sensing Power Switch

This circuit will automatically switch on several mains-powered "slave" loads when a "master" load is turned on. For example, it will switch on the amplifier and CD player in a stereo system when the receiver is turned on. It works by sensing the current draw of the "master" device through a low value high wattage resistor using a comparator. The output of that comparator then switches on the "slave" relay. The circuit can be built into a power bar, extension cord or power center to provide a convenient set of "smart" outlets that switch on when the master appliance is powered (turn on the computer monitor and the computer, printer and other peripherals come on as well).

Schematic:

Parts: 

Part	Total Qty.	Description
C1, C3	2	10uF 35V Electrolytic Capacitor
C2	1	1uF 35V Electrolytic Capacitor
R1	1	0.1 Ohm 10W Resistor
R2	1	27K 1/2W Resistor
R3, R4	1	1K 1/4W Resistor
R5	1	470K 1/4W Resistor
R6	1	4.7K 1/2W Resistor
R7	1	10K 1/4W Resistor
D1, D2, D4	3	1N4004 Rectifier Diode
D3	1	1N4744 15V 1 Watt Zener Diode
U1	1	LM358N Dual Op Amp IC
Q1	1	2N3904 NPN Transistor
K1	1	Relay, 12VDC Coil, 120VAC 10A Contacts
S1	1	SPST Switch 120AVC, 10A
MISC	1	Board, Wire, Socket For U1, Case, Mains Plug, Socket

 Notes:

1. This circuit is designed for 120V operation. For 240V operation, resistors R2 and R6 will need to be changed.
2. A maximum of 5A can be used as the master unless the wattage of R1 is increased
3. S1 provides a manual bypass switch.
4. THis circuit is not isolated from the mains supply. Because of this, you must exercise extreme caution when working around the circuit if it is plugged in.

12V to 120V Inverter

Schematic:

Parts:

Part	Total Qty.	Description
C1, C2	2	68 uf, 25 V Tantalum Capacitor
R1, R2	2	10 Ohm, 5 Watt Resistor
R3, R4	2	180 Ohm, 1 Watt Resistor
D1, D2	2	HEP 154 Silicon Diode
Q1, Q2	2	2N3055 NPN Transistor (see "Notes")
T1	1	24V, Center Tapped Transformer (see "Notes")
MISC	1	Wire, Case, Receptical (For Output)

Notes:

1. Q1 and Q2, as well as T1, determine how much wattage the inverter can supply. With Q1,Q2=2N3055 and T1= 15 A, the inverter can supply about 300 watts. Larger transformers and more powerful transistors can be substituted for T1, Q1 and Q2 for more power.
2. The easiest and least expensive way to get a large T1 is to re-wind an old microwave transformer. These transformers are rated at about 1KW and are perfect. Go to a local TV repair shop and dig through the dumpster until you get the largest microwave you can find. The bigger the microwave the bigger transformer. Remove the transformer, being careful not to touch the large high voltage capacitor that might still be charged. If you want, you can test the transformer, but they are usually still good. Now, remove the old 2000 V secondary, being careful not to damage the primary. Leave the primary in tact. Now, wind on 12 turns of wire, twist a loop (center tap), and wind on 12 more turns. The guage of the wire will depend on how much current you plan to have the transformer supply. Enamel covered magnet wire works great for this. Now secure the windings with tape. Thats all there is to it. Remember to use high current transistors for Q1 and Q2. The 2N3055's in the parts list can only handle 15 amps each.
3. Remember, when operating at high wattages, this circuit draws huge amounts of current. Don't let your battery go dead :-).
4. Since this project produces 120 VAC, you must include a fuse and build the project in a case.
5. You must use tantalum capacitors for C1 and C2. Regular electrolytics will overheat and explode. And yes, 68uF is the correct value. There are no substitutions.
6. This circuit can be tricky to get going. Differences in transformers, transistors, parts substitutions or anything else not on this page may cause it to not function.
7. If you want to make 220/240 VAC instead of 120 VAC, you need a transformer with a 220/240 primary (used as the secondary in this circuit as the transformer is backwards) instead of the 120V unit specified here. The rest of the circuit stays the same. But it takes twice the current at 12V to produce 240V as it does 120V.
8. Check out this forum topic to answer many of the most commonly asked questions about this circuit: 12 - 120V Inverter Again. It covers the most common problems encountered and has some helpful suggestions.

AC Motor Speed Controller

This AC motor speed controller can handle most universal type (brushed) AC motors and other loads up to about 250W. It works in much the same was a light dimmer circuit; by chopping part of the AC waveform off to effectively control voltage. Because of this functionality, the circuit will work for a wide variety of loads including incandescent light bulbs, heating elements, brushed AC motors and some transformers. The circuit tries to maintain a constant motor speed regardless of load so it is also ideal for power tools. Note that the circuit can only control brushed AC motors. Inductive motors require a variable frequency control.

Schematic:

Parts:

Part	Total Qty.	Description
1	1	27K 1W Resistor
R2	1	10K 1/4W Resistor
R3	1	100K 1/4W Resistor
R4	1	33K 1/4W Resistor
R5	1	2.2K 1/4W Resistor
R6	1	1K 1/4W Resistor
R7	1	60K Ohm 1/4W Resistor
R8	1	3K Linear Taper Trim Pot
R9	1	5K Linear Taper Pot
R10	1	4.7K Linear Taper Trim Pot
R11	1	3.3K 1/4W Resistor
R12	1	100 Ohm 1/4W Resistor
R13	1	47 Ohm 1W Resistor (See Notes)
C1, C3	2	0.1uF Ceramic Disc Capacitor
C2	1	100uF 50V Electrolytic Capacitor
D1	1	6V Zener Diode
Q1	1	2N2222 NPN Transistor
SCR1	1	ECG5400
TR1	1	TRIAC (See Notes)
U1	1	DIAC Opto-Isolator (See Notes)
BR1, BR2	2	5A 50V Bridge Rectifier
T1	1	Transformer (See Notes)
MISC	1	PC Board, Case, Line Cord, Socket For U1, Heatsinks

Notes:

1. TR1 must be chosen to match the requirements of the load. Most generic TRIACs with ratings to support your load will work fine in this circuit. If you find a TRIAC that works well, feel free to leave a comment.
2. U1 must be chosen to match the ratings of TR1. Most generic DIAC based opto-isolators will work fine. If you have success with a specific part, feel free to leave a comment.
3. T1 is any small transformer with a 1:10 turns ratio. The circuit is designed to run on 120V so a 120V to 12V transformer will work. Alternately, you can wind T1 on a transformer core using a primary of 25 turns, a secondary of 200 turns, and 26 gauge magnet wire.
4. R9 is used to adjust motor speed. R10 is a trim pot used to fine tune the governing action of the circuit. R8 fine tunes the feedback circuit to adjust for proper voltage at the gate of SCR1. It should be adjusted to just past the minimum point at which the circuit begins to operate.
5. R13 must be chosen to match the load. Generally, larger loads will require a smaller value.
6. Since this circuit is not isolated from mains, it must be built in an insulated case.

Model Railway Signal Project

  Download PDF version of this page 
A magnet under the train operates reed switches positioned on the track. The first reed switch changes the signal to red as the train passes, then further along the track a second reed switch changes the signal back to green ready for the next train. The isolated section of track just in front of the signal is switched off by the relay when the signal is red so a train will stop automatically at the red signal.
This project uses a 555 bistable circuit.

Parts Required

• resistors: 1k ×2, 33k ×2
• capacitors: 220µF
• 1N4001 diode
• 1N4148 diode
• red LED (3mm best)
• green LED (3mm best)
• 555 timer IC
• 8-pin DIL socket for IC
• push-switch ×2
• reed switch ×2
• relay SPCO 12V coil
• miniature magnet - each locomotive needs one
• stripboard 11 rows × 24 holes

Stripboard layout

Track connections

• Connect the reed switches to push-switches A and B (see the stripboard layout).
• The switches can be held in place between the rails with a small piece of blu tac.
• Connect the track wires to the COM and NC contacts of the relay.
• When soldering to the track make sure you solder to the outside of the rail.
• Each locomotive will need a miniature magnet glued to its underside - test first with blu tac, but superglue is probably best once you are sure it is in the correct position.
• Note that railway signals have red at the bottom, unlike road traffic lights where red is at the top.

Circuit diagram

555 and 556 Timer Circuits

Introduction

Example circuit symbol (above)Actual pin arrangements (below)

The 8-pin 555 timer must be one of the most useful ICs ever made and it is used in many projects. With just a few external components it can be used to build many circuits, not all of them involve timing!A popular version is the NE555 and this is suitable in most cases where a '555 timer' is specified. The 556 is a dual version of the 555 housed in a 14-pin package, the two timers (A and B) share the same power supply pins. The circuit diagrams on this page show a 555, but they could all be adapted to use one half of a 556.
Low power versions of the 555 are made, such as the ICM7555, but these should only be used when specified (to increase battery life) because their maximum output current of about 20mA (with a 9V supply) is too low for many standard 555 circuits. The ICM7555 has the same pin arrangement as a standard 555.
The circuit symbol for a 555 (and 556) is a box with the pins arranged to suit the circuit diagram: for example 555 pin 8 at the top for the +Vs supply, 555 pin 3 output on the right. Usually just the pin numbers are used and they are not labelled with their function.
The 555 and 556 can be used with a supply voltage (Vs) in the range 4.5 to 15V (18V absolute maximum).
Standard 555 and 556 ICs create a significant 'glitch' on the supply when their output changes state. This is rarely a problem in simple circuits with no other ICs, but in more complex circuits a smoothing capacitor (eg 100µF) should be connected across the +Vs and 0V supply near the 555 or 556.

Inputs of 555/556

Trigger input: when < ¹/₃ Vs ('active low') this makes the output high (+Vs). It monitors the discharging of the timing capacitor in an astable circuit. It has a high input impedance > 2M.Threshold input: when > ²/₃ Vs ('active high') this makes the output low (0V)*. It monitors the charging of the timing capacitor in astable and monostable circuits. It has a high input impedance > 10M. 
* providing the trigger input is > ¹/₃ Vs, otherwise the trigger input will override the threshold input and hold the output high (+Vs).
Reset input: when less than about 0.7V ('active low') this makes the output low (0V), overriding other inputs. When not required it should be connected to +Vs. It has an input impedance of about 10k.
Control input: this can be used to adjust the threshold voltage which is set internally to be ²/₃ Vs. Usually this function is not required and the control input is connected to 0V with a 0.01µF capacitor to eliminate electrical noise. It can be left unconnected if noise is not a problem.
The discharge pin is not an input, but it is listed here for convenience. It is connected to 0V when the timer output is low and is used to discharge the timing capacitor in astable and monostable circuits.

---

Output of 555/556

The output of a standard 555 or 556 can sink and source up to 200mA. This is more than most ICs and it is sufficient to supply many output transducers directly, including LEDs (with a resistor in series), low current lamps, piezo transducers, loudspeakers (with a capacitor in series), relay coils (with diode protection) and some motors (with diode protection). The output voltage does not quite reach 0V and +Vs, especially if a large current is flowing.To switch larger currents you can connect a transistor.
The ability to both sink and source current means that two devices can be connected to the output so that one is on when the output is low and the other is on when the output is high. The top diagram shows two LEDs connected in this way. This arrangement is used in the Level Crossing project to make the red LEDs flash alternately.

Loudspeakers

A loudspeaker (minimum resistance 64) may be connected to the output of a 555 or 556 astable circuit but a capacitor (about 100µF) must be connected in series. The output is equivalent to a steady DC of about ½Vs combined with a square wave AC (audio) signal. The capacitor blocks the DC, but allows the AC to pass as explained in capacitor coupling.Piezo transducers may be connected directly to the output and do not require a capacitor in series.

Relay coils and other inductive loads

Like all ICs, the 555 and 556 must be protected from the brief high voltage 'spike' produced when an inductive load such as a relay coil is switched off. The standard protection diode must be connected 'backwards' across the the relay coil as shown in the diagram.However, the 555 and 556 require an extra diode connected in series with the coil to ensure that a small 'glitch' cannot be fed back into the IC. Without this extra diode monostable circuits may re-trigger themselves as the coil is switched off! The coil current passes through the extra diode so it must be a 1N4001 or similar rectifier diode capable of passing the current, a signal diode such as a 1N4148 is usually not suitable.

555/556 Astable

555 astable output, a square wave (Tm and Ts may be different)

555 astable circuit

An astable circuit produces a 'square wave', this is a digital waveform with sharp transitions between low (0V) and high (+Vs). Note that the durations of the low and high states may be different. The circuit is called an astable because it is not stable in any state: the output is continually changing between 'low' and 'high'.The time period (T) of the square wave is the time for one complete cycle, but it is usually better to consider frequency (f) which is the number of cycles per second.

T = 0.7 × (R1 + 2R2) × C1   and  f =	1.4
	(R1 + 2R2) × C1

T   = time period in seconds (s)
f    = frequency in hertz (Hz)
R1 = resistance in ohms ()
R2 = resistance in ohms ()
C1 = capacitance in farads (F)
The time period can be split into two parts: T = Tm + Ts
Mark time (output high): Tm = 0.7 × (R1 + R2) × C1
Space time (output low): Ts  = 0.7 × R2 × C1
Many circuits require Tm and Ts to be almost equal; this is achieved if R2 is much larger than R1.
For a standard astable circuit Tm cannot be less than Ts, but this is not too restricting because the output can both sink and source current. For example an LED can be made to flash briefly with long gaps by connecting it (with its resistor) between +Vs and the output. This way the LED is on during Ts, so brief flashes are achieved with R1 larger than R2, making Ts short and Tm long. If Tm must be less than Ts a diode can be added to the circuit as explained under duty cycle below. 

Choosing R1, R2 and C1

555 astable frequencies
C1	R2 = 10k R1 = 1k	R2 = 100k R1 = 10k	R2 = 1M R1 = 100k
0.001µF	68kHz	6.8kHz	680Hz
0.01µF	6.8kHz	680Hz	68Hz
0.1µF	680Hz	68Hz	6.8Hz
1µF	68Hz	6.8Hz	0.68Hz
10µF	6.8Hz	0.68Hz (41 per min.)	0.068Hz (4 per min.)

R1 and R2 should be in the range 1k to 1M. It is best to choose C1 first because capacitors are available in just a few values.

• Choose C1 to suit the frequency range you require (use the table as a guide).
• Choose R2 to give the frequency (f) you require. Assume that R1 is much smaller than R2 (so that Tm and Ts are almost equal), then you can use:
  

  R2 =	0.7
  	f × C1

• Choose R1 to be about a tenth of R2 (1k min.) unless you want the mark time Tm to be significantly longer than the space time Ts.
• If you wish to use a variable resistor it is best to make it R2.
• If R1 is variable it must have a fixed resistor of at least 1k in series
  (this is not required for R2 if it is variable).

Astable operation

With the output high (+Vs) the capacitor C1 is charged by current flowing through R1 and R2. The threshold and trigger inputs monitor the capacitor voltage and when it reaches ²/₃Vs (threshold voltage) the output becomes low and the discharge pin is connected to 0V.The capacitor now discharges with current flowing through R2 into the discharge pin. When the voltage falls to ¹/₃Vs (trigger voltage) the output becomes high again and the discharge pin is disconnected, allowing the capacitor to start charging again.
This cycle repeats continuously unless the reset input is connected to 0V which forces the output low while reset is 0V.
An astable can be used to provide the clock signal for circuits such as counters.
A low frequency astable (< 10Hz) can be used to flash an LED on and off, higher frequency flashes are too fast to be seen clearly. Driving a loudspeaker or piezo transducer with a low frequency of less than 20Hz will produce a series of 'clicks' (one for each low/high transition) and this can be used to make a simple metronome.
An audio frequency astable (20Hz to 20kHz) can be used to produce a sound from a loudspeaker or piezo transducer. The sound is suitable for buzzes and beeps. The natural (resonant) frequency of most piezo transducers is about 3kHz and this will make them produce a particularly loud sound. 

Duty cycle

The duty cycle of an astable circuit is the proportion of the complete cycle for which the output is high (the mark time). It is usually given as a percentage.For a standard 555/556 astable circuit the mark time (Tm) must be greater than the space time (Ts), so the duty cycle must be at least 50%:

Duty cycle  =	Tm	=	R1 + R2
	Tm + Ts		R1 + 2R2

555 astable circuit with diode across R2

To achieve a duty cycle of less than 50% a diode can be added in parallel with R2 as shown in the diagram. This bypasses R2 during the charging (mark) part of the cycle so that Tm depends only on R1 and C1:
Tm = 0.7 × R1 × C1   (ignoring 0.7V across diode)
Ts  = 0.7 × R2 × C1   (unchanged)

Duty cycle with diode  =	Tm	=	R1
	Tm + Ts		R1 + R2

Use a signal diode such as 1N4148.

555/556 Monostable

555 monostable output, a single pulse

555 monostable circuit with manual trigger

A monostable circuit produces a single output pulse when triggered. It is called a monostable because it is stable in just one state: 'output low'. The 'output high' state is temporary.The duration of the pulse is called the time period (T) and this is determined by resistor R1 and capacitor C1:

time period, T = 1.1 × R1 × C1

T   = time period in seconds (s)
R1 = resistance in ohms ()
C1 = capacitance in farads (F)
The maximum reliable time period is about 10 minutes.
Why 1.1? The capacitor charges to ²/₃ = 67% so it is a bit longer than the time constant (R1 × C1) which is the time taken to charge to 63%.

• Choose C1 first (there are relatively few values available).
• Choose R1 to give the time period you need. R1 should be in the range 1k to 1M, so use a fixed resistor of at least 1k in series if R1 is variable.
• Beware that electrolytic capacitor values are not accurate, errors of at least 20% are common.
• Beware that electrolytic capacitors leak charge which substantially increases the time period if you are using a high value resistor - use the formula as only a very rough guide!
  For example the Timer Project should have a maximum time period of 266s (about 4½ minutes), but many electrolytic capacitors extend this to about 10 minutes!

Monostable operation

The timing period is triggered (started) when the trigger input (555 pin 2) is less than ¹/₃ Vs, this makes the output high (+Vs) and the capacitor C1 starts to charge through resistor R1. Once the time period has started further trigger pulses are ignored.The threshold input (555 pin 6) monitors the voltage across C1 and when this reaches ²/₃ Vs the time period is over and the output becomes low. At the same time discharge (555 pin 7) is connected to 0V, discharging the capacitor ready for the next trigger.
The reset input (555 pin 4) overrides all other inputs and the timing may be cancelled at any time by connecting reset to 0V, this instantly makes the output low and discharges the capacitor. If the reset function is not required the reset pin should be connected to +Vs. 

Power-on reset or trigger circuit

Power-on reset or trigger

It may be useful to ensure that a monostable circuit is reset or triggered automatically when the power supply is connected or switched on. This is achieved by using a capacitor instead of (or in addition to) a push switch as shown in the diagram.The capacitor takes a short time to charge, briefly holding the input close to 0V when the circuit is switched on. A switch may be connected in parallel with the capacitor if manual operation is also required.
This arrangement is used for the trigger in the Timer Project. 

Edge-triggering

edge-triggering circuit

If the trigger input is still less than ¹/₃ Vs at the end of the time period the output will remain high until the trigger is greater than ¹/₃ Vs. This situation can occur if the input signal is from an on-off switch or sensor.The monostable can be made edge triggered, responding only to changes of an input signal, by connecting the trigger signal through a capacitor to the trigger input. The capacitor passes sudden changes (AC) but blocks a constant (DC) signal. For further information please see the page on capacitance. The circuit is 'negative edge triggered' because it responds to a sudden fall in the input signal.
The resistor between the trigger (555 pin 2) and +Vs ensures that the trigger is normally high (+Vs).

555/556 Bistable (flip-flop) - a memory circuit

555 bistable circuit

The circuit is called a bistable because it is stable in two states: output high and output low. It is also known as a 'flip-flop'.It has two inputs:

• Trigger (555 pin 2) makes the output high.
  Trigger is 'active low', it functions when < ¹/₃ Vs.
• Reset (555 pin 4) makes the output low.
  Reset is 'active low', it resets when < 0.7V.

The power-on reset, power-on trigger and edge-triggering circuits can all be used as described above for the monostable.

555/556 Inverting Buffer (Schmitt trigger) or NOT gate

555 inverting buffer circuit (a NOT gate)

NOT gate symbol

The buffer circuit's input has a very high impedance (about 1M) so it requires only a few µA, but the output can sink or source up to 200mA. This enables a high impedance signal source (such as an LDR) to switch a low impedance output transducer (such as a lamp).It is an inverting buffer or NOT gate because the output logic state (low/high) is the inverse of the input state:

• Input low (< ¹/₃ Vs) makes output high, +Vs
• Input high (> ²/₃ Vs) makes output low, 0V

When the input voltage is between ¹/₃ and ²/₃ Vs the output remains in its present state. This intermediate input region is a deadspace where there is no response, a property calledhysteresis, it is like backlash in a mechanical linkage. This type of circuit is called a Schmitt trigger.If high sensitivity is required the hysteresis is a problem, but in many circuits it is a helpful property. It gives the input a high immunity to noise because once the circuit output has switched high or low the input must change back by at least ¹/₃ Vs to make the output switch back.