This repeating timer can be adjusted to work only when it's hot. It lets you set the level of temperature at which it will start to run. The length of time the outputs last - and the length of the intervals in between - are also adjustable.

COMMENTS SUGGESTIONS	Repeating Timer No.5 - Testing Your Circuit	MORE TIMER CIRCUITS

Timer No.5 Schematic	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

Repeat Timer No.5 - Support Material

Do not use the "on-board" relay to switch mains voltage. The board's layout does not offer sufficient isolation between the relay contacts and the low-voltage components. If you want to switch mains voltage - mount a suitably rated relay somewhere safe - Away From The Board.

Introduction

The prototype of the Repeating Timer No5 was built using only the Stripboard Layout as a guide. So - if you have faithfully reproduced that layout - you will have a working circuit.

Once you're satisfied that your layout is correct - and you have made a careful and thorough check of the underside of the board - it's time to power-up the circuit and test its operation. This is always an anxious moment. If you construct a lot of circuits - you might consider building the Current Limiting Power Supply - or alternatively - you could add the Simple Current Limiter to your existing PSU. Both will let you set an upper limit on the amount of current supplied to your circuit - and so protect it from any serious damage.

Setup

Click Here For A Photograph Of The Prototype

Details of How to Prepare The
Repeating Timer No.5 - For Testing

A resistor and a couple of LEDs are all that's needed to demonstrate that the relay is working properly. 

Connect a 2k2 resistor between the  Pole and the Positive terminals. 

Connect two LEDs from the N/O and N/C terminals to the Negative terminal. 

I used a red and a green LED in the diagram - so that I can distinguish between them.

But you can use whatever colour LEDs you have available. 

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Next - connect the 12-volts DC to the input terminals. 

Pay particular attention to the polarity of the supply. 

Note that the positive connection goes to the top terminal. 

Before you switch on the power - turn R2, R3 & R7 all the way to the left.

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When you switch on the power - the relay may or may not energize immediately.

It all depends on the switching characteristics of the actual gates you are using.

That is - it depends on whether your 4011 powers up with pin 3 high or low.

If the relay doesn't energize immediately - wait 30 to 60 seconds.

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When the relay energizes - the green LED should switch off - and the red LED should light.

It should remain lit for about 30 to 60 seconds.

Then the relay should de-energize - the red LED should turn off - and the green LED should light.

It should remain lit for about 30 to 60 seconds.

Then the relay should energize again - and the red LED should light.

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The circuit should continue switching back and forward - between the red and green LEDs - at 30 to 60 second intervals.

It's likely that the two intervals will be different lengths.

This is not a fault. It's caused by the switching points of the actual gates you're using.

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Allow the switching to continue - until you're satisfied that your circuit is working reliably.

Then turn (R3) a little to the right - see the photo of the prototype.

The relay should continue to energize for 30 to 60 seconds - as before.

But now - it should remain de-energized for a longer period - say about 3 to 5-minutes.

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When you're satisfied your circuit is working as described - turn R3 all the way back to the left.

Then turn R2 a little way to the right.

Now - the relay should remain energized for - about 3 to 5-minutes.

And remain de-energized for 30 to 60 seconds

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When you're satisfied your circuit is working as described - turn R2 all the way back to the left.

This should restore the shorter on/off cycle.

Let it run for a few cycles - then turn R7 all the way to the right.

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This should stop the timer.

If the relay is de-energized - it should remain de-energized.

If the relay is energized - it should stay that way for the balance of its 30 to 60 second time period.

Then it should de-energize - and remain de-energized.

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When you're satisfied that the timer has stopped completely - the next step is to test the temperature control.

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The thermistor has a small mass - and its temperature will change fairly rapidly.

It's also sensitive to cooling air currents.

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To slow things down - and to produce more stable temperatures - cover the thermistor with a couple of layers of cloth.

It's not necessary to wrap the cloth around the thermistor.

Just place the thermistor on the bench - and lay the cloth on top.

The bulb of your thermometer should also be under the cloth - next to the thermistor.

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Use your desk lamp to raise the temperature of the thermistor.

The lamp shouldn't be placed too close to the fabric. You don't want to set it on fire.

With a 60-watt lamp - a distance of say 10 inches (250mm) should be sufficient.

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Raise the temperature of the thermistor by about five degrees centigrade or ten degrees Fahrenheit.

You don't have to be too precise.

It just needs to be significantly higher than the ambient temperature.

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When it reaches the desired level - continue to apply the heat. 

But turn R7 slowly to the left.

Do this in a series of small discrete steps.

You're looking for the point where the relay just begins to operate again.

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When you've found it - wait for the temperature to rise by a further five degrees centigrade or ten degrees Fahrenheit.

Them turn off the lamp.

The timer should continue to run - and the temperature should start to fall. 

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Because of the insulation provided by the cloth - the temperature should fall slowly.

When it reaches the level you've set with R7 - the timer will stop running.

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Allow the temperature to fall by a few more degrees.

Then turn on the lamp - and warm the thermistor again - slowly.

Observe the thermometer.

When the relay energizes - the thermometer reading will tell you the temperature above which the timer will run.

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Try setting different levels of temperature.

Monitor the voltage on pin 1.

The instant before the relay energizes - the voltage reading on pin 1 is the switching point of the gate.

Do not confuse it with the higher reading - after the relay has energized.

At that time - pin 1 is being held artificially high - by D4.

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Adjust R7 - only while the relay is de-energized.

If you reduce the voltage on pin 1 - it will increase the temperature at which the timer begins to run.

And - if you increase the voltage on pin 1 - it will reduce the temperature at which the timer begins to run.

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This technique will give you a certain amount of feel for how the temperature is set.

It will also let you set R7 at roughly the right value.

But the crude insulation of the thermistor - and the artificial and relatively fast temperature changes caused by the lamp - mean that you'll still have to make fine adjustments when the circuit is installed in its actual working environment. 


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Sample Readings

The following readings will give you some idea of what to expect. They are taken from the prototype. They apply to the Cmos 4011. And they were made using a cheap digital multimeter and a cheap thermometer. They are intended for guidance only. You should NOT expect to get identical readings.

My thermistor measured 4.45K at 25慢 (77慚). My supply voltage measured 12v4. And the Pin 1 switching point was measured at just over 6v9. With R7 set to 5k - the timer worked at temperatures above about 21.5慢 (71慚).

If You Find a Problem

If - in the course of the test - you find that something is not working properly - then a careful inspection of the relevant area of the circuit board should turn up the cause of the problem. Where you've cut the board to size - look for small loose strands of copper left behind by the saw. Check the board for short-circuits caused by component leads touching each other. If an LED is not lighting - check that it's connected the right way round. It can also happen that the stripboard itself is faulty. I have seen cases where the copper tracks have not been completely severed from one another during manufacture.

If you've built your circuit using the specified components - and you've followed the step-by-step construction guide described on the Repeating Timer No.5 - Support Page - then the chances are that any bug will be caused by something minor - a component connected the wrong way round - a missing or unwanted solder bridge - an incomplete cut in the track etc.

If you can't see anything obvious then adopt a systematic approach to faultfinding. Begin by double-checking that all of the cuts in the tracks have been made, that they are all - In The Right Place - and that they sever the track completely. Use a magnifying glass. It only takes the smallest strand of copper to cause a problem. If you backlight the board during the examination - it makes them easier to spot.

When you're satisfied that the tracks have been severed in all the right places, check that you have made - and correctly placed - all of the solder bridges. Mark each bridge with a felt-tip pen - or something similar - so that it can be easily identified later.

Next - carefully examine the full length of each track. Look for unwanted solder bridges. Your felt-tip markings will tell you which ones should be there - and help you identify any that shouldn't be there. Again, if you backlight the board during the examination - it makes potential problem areas easier to spot.

If all else fails and you still haven't found the cause of the problem - work your way through the assembly instructions on the Support Page. Check each individual component and link - to make sure that it's present and correctly positioned.

Print out the drawings and mark off the components as you go. Pay particular attention to the orientation of the diodes, the transistor and electrolytic capacitors. Take your time and examine each individual component carefully.

Repeating Timer No.5 - Support Material

Repeating Timer No.5 Schematic	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

Repeating Timer No.5 - Testing Your Circuit

Introduction

Setup

Sample Readings

If You Find a Problem