This simple circuit will energize a relay at regular intervals. Both the length of time the relay remains energized - and the length of the intervals in between - are independently adjustable.

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Circuit Description Click Here For A Photograph Of The Prototype. This circuit uses a Cmos 4001. The 4001 contains four two-input NOR gates. All four gates share the same two connections to the power supply - at pins 14 & 7. But apart from that - they operate entirely independently of one another. ============ The state of the relay is controlled by the state of pin 3. When pin 3 is high - it's connected internally to the positive line. And it supplies base current to Q1 - through R1. R1 limits the size of that current to under 3mA. ============ The base current switches the transistor on. The transistor connects the negative side of the relay coil to ground. And the relay energizes. ============ When pin 3 is low - it's connected internally to the negative line. Pin 3 connects the base of Q1 to ground - through R1. This switches the transistor off. The transistor disconnects the negative side of the relay coil from ground. And the relay de-energizes. ============ The state of pin 3 is controlled by pin 11 - through gates 1 & 2. Gates 1 & 2 each have their two input pins connected together. This means that they are being used as simple inverters. ============ When pin 11 goes high - it takes pins 5 & 6 high. So pin 4 goes low. Pin 4 takes pins 1 & 2 low. So pin 3 goes high. ============ A similar sequence of inversions occurs when pin 11 goes low. The end result is that - the state of pin 3 - will always be an exact copy of the state of pin 11. ============ Gates 1 & 2 are in fact spare. They are not really necessary to the operation of the circuit. If you have another use for gates 1 & 2 - connect R1 directly to pin 11. ============ Gates 3 & 4 are connected together to form a monostable. For a detailed account of how the monostable works - click on the link below the diagram. ============ Basically - a monostable is a circuit that provides an output for a certain length of time - and then switches itself off. ============ The output from the monostable is at pin 11. And the length of time pin 11 remains high - depends on the values of R3 & C2. ============ With a 470uF capacitor - pin 11 will remain high for up to about 30 minutes. While pin 11 remains high - pin 3 will remain high also. And while pin 3 remains high - the relay will remain energized. In other words - the length of the "ON" time - is controlled by the values of R3 & C2. ============ The length of the "OFF" time is controlled by the values of R4 & C3. Again - with a 470uF capacitor - the relay will remain de-energized for up to about 30 minutes. ============ To begin with - C3 is in a discharged state. So the voltage on pin 8 is zero. In other words - pin 8 is low. ============ As R4 charges C3 - the voltage on pin 8 rises. Eventually - the voltage across C3 takes pin 8 high. And this triggers the monostable. ============ When C3 triggers the monostable - pins 12 & 13 go low. This is because C2 is in a discharged state. When pins 12 & 13 go low - pin 11 goes high. Pin 11 takes pin 3 high - through gates 1 & 2. And the relay energizes. ============ At this point - R3 begins to charge C2. As it does so - the voltage on pins 12 & 13 rises. Eventually - the voltage across C2 will take pins 12 & 13 high. Then - pin 11 goes low. Pin 11 takes pin 3 low - through gates 1 & 2. And the relay de-energizes. ============ It will remain de-energized until the next time C3 takes pin 8 high. Then - the monostable will trigger - pin 11 will go high - and the relay will energize once more. ============ We want both the "ON" timer - and the "OFF" timer - to repeat continuously. This means that when each has finished its job - it must be reset. In other words - C2 and C3 must be discharged at the appropriate time. ============ A capacitor is completely discharged - when there is no voltage drop across its plates. That is - when the voltage on its positive plate is exactly the same as the voltage on its negative plate. ============ A capacitor with zero volts on both plates is fully discharged. But it doesn't have to be zero volts. A capacitor with 12 volts on both plates is also fully discharged. ============ When C3 is fully discharged - it has zero volts on both of its plates. But - when C2 is fully discharged - it has 12 volts on each plate. ============ Both capacitors are discharged by pin 10. When the monostable is triggered - pin 10 goes low. While pin 10 is low - it's connected internally to the negative line. And C3 discharges into the low pin 10 - through R2 & D3. This resets the "OFF" timer. ============ When the "ON" time period expires - pin 10 goes high. While pin 10 is high - it's connected internally to the positive line. And it discharges C2 - by applying a positive charge to the capacitor's negative plate. This resets the monostable. That is - it resets the "ON" timer. ============ Because pin 10 discharges C3 - the "OFF" timer is always ready to start again - when the "ON" time expires. And because pin 10 discharges C2 - the "ON" timer is always ready to start again - when the "OFF" time expires. This is why the timer repeats. ============ I used an SPCO/SPDT relay in the prototype - but you can use a multi-pole relay if it suits your application. The circuit will work from a minimum of 5-volts up to a maximum of 15-volts. You just need to make sure that the coil of your relay will operate at the voltage you are using. Also - the relay coil shouldn't draw more than about 50mA. Otherwise the transistor might be overloaded. ============ With a 12-volt supply - I specified a minimum coil resistance of about 270 ohms. This gives a maximum current of 12÷270 = 45 mA. 45mA is well within the limits of the BC547. If your relay coil needs more than about 50mA - upgrade Q1 to a BC337. ============ There's nothing special about the BC547. Any small transistor with a DC gain (hfe) greater than 100 and an Ic(max) of at least 100mA should do. But remember that the pin configuration of your transistor may be different from that of the BC547. ============ Coils of all sorts give off high reverse-voltage spikes that will destroy Cmos ICs. Always connect a diode across relay coils - and across any other device that might contain a coil. ============ The high voltage spikes are generated by the relay coil - when it de-energizes - and its magnetic field collapses. It's rather like the high voltage spark - given off by a car's ignition coil - when the points open. It's polarity is opposite to that of the supply voltage. ============ Note that the cathode of D1 - the side with the bar - is connected to the positive terminal of the coil. The small arrowhead indicates the direction in which the diode will conduct. Thus, any reverse-voltage spikes will be short-circuited at source - before they can do any damage. ============ D2 Creates A One-Way Path. It allows a high pin 10 to charge C2 But - it prevents a low pin 10 from taking pins 12 & 13 low. ============ Although a Cmos 4001 gate is a digital device - there are times when its output is more akin to analogue. With the given values of C2 & R3 - the voltage on pins 12 & 13 rises slowly. And it may hover around the gate's switching point - for a significant period of time. ============ During this period - the input pins are neither high nor low. And - as a result - the output pin is also neither high nor low. This period of uncertainty can last a significant length of time. And - during it - the relay may rattle or buzz. ============ D2 shortens this period of uncertainty. When pins 12 & 13 start to go high - pin 11 starts to go low. This causes pin 10 to start to go high. And the high pin 10 starts to charge C2. This takes pins 12 & 13 further high. ============ In effect - there's a positive feedback loop. At the crucial moment - pin 10 takes over from the slow R3 And quickly completes the charging of C2. ============ D3 Creates A One-Way Path. It allows C3 to discharge rapidly - into a low pin 10. But it prevents a high pin 10 from charging C3. ============ R2 slows down the speed at which C3 discharges. In so doing - it increases the the length of the monostable's trigger pulse. ============ When C3 takes pin 8 high - it triggers the ON timer. The problem is that the instant C3 takes pin 8 high - pin 10 goes low. Then the low pin 10 discharges C3. And - almost immediately - pin 8 will go low again. ============ A very brief high - at pin 8 - should be enough to trigger the monostable. But just in case - I added R2 to slow down the rate at which C3 discharges. The presence of R2 means that pin 8 will stay high for a little bit longer. And so provide the monostable with a more substantial trigger pulse. ============ Although static electricity can destroy Cmos ICs - they are extremely reliable in operation. Most modern versions will survive a fair degree of abuse. But it's worth taking a couple of basic precautions. Avoid touching the pins - and don't overheat them with the soldering iron. A socket will reduce the chances of damaging the IC - and will make it easier to replace if necessary. If you're using a socket - always switch-off the power before inserting or removing the IC. Also - check that the IC is the right way round - and that all of the pins are correctly inserted into the socket. Sometimes - instead of entering the socket - a pin will curl up underneath the IC. ============

Learn More About How A Monostable Works

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Construction Guide

Click here if you're new to constructing stripboard projects.

The prototype of the timer was built using only the Stripboard Layout as a guide. So - if you reproduce the layout - you will have a working timer. Details of how to Test Your Finished Circuit Board are also provided.

The terminals are a good set of reference points. To fit them - you may need to enlarge the holes slightly. Then turn the board over and use a felt-tip pen to mark the 16 places where the tracks are to be cut. Before you cut the tracks - use the "actual size" drawing to Check That The Pattern is Correctly Marked .

Actual Size

When you're satisfied that the pattern is right - cut the tracks. If you don't have the proper track-cutting tool - then a 6 to 8mm drill-bit will do. Just use the drill-bit as a hand tool - there's no need for a drilling machine. Make sure that the copper strip is cut all the way through. Sometimes a small strand of copper remains at the side of the cut and this will cause malfunction. Use a magnifying glass - and backlight the board. It only takes the smallest strand of copper to cause a problem.

Next make and fit the Nine Wire Links - fit the two presets - and fit the two fixed resistors. For the links - I used bare copper wire on the component side of the board. Telephone cable is suitable - the single stranded variety used indoors to wire telephone sockets. Stretching the core slightly will straighten it - and also allow the insulation to slip off.




Then fit the three diodes and the transistor. Pay particular attention to the orientation of the diodes. See the Photograph Of The Prototype. Note that D3 is facing downwards.




Next, fit the three capacitors - the relay - and the IC socket. Pay particular attention to the orientation of the electrolytic capacitors. Note that each has its negative terminal - the side with the stripe - facing downwards.

Turn the board over and examine the underside carefully - to make sure that there are no unwanted solder bridges or other connections between the tracks. If you backlight the board during the examination - it makes potential problem areas easier to spot. When you're satisfied that everything is in order - add the 7 solder bridges. These are just small blobs of solder. They connect two adjacent tracks together. They're like very short wire links.




Finish off by inserting the Cmos IC into the socket. Pin 1 of the IC should be in the top left-hand corner. Check that all 14 pins have entered the socket. Sometimes - instead of entering the socket - a pin will curl up under the IC.