by Harry Lythall - SM0VPO

I have received a request for a 5-seconds timer. The logic function is that the output of the timer changes from LOW to HIGH five seconds after the power is applied. Instead of writing the answer as E-mail I thought that all could benefit from a posting. So here is is.

When applying the power the capacitor charges up and after a few seconds reaches about half the supply volts. If the potentiometer is to about half way then the slider is also at about half the supply volts. The OP-Amp compares the two voltages and the output changes state when the capacitor voltage rises above the preset potentiometer voltage. The setting of the potentiometer will vary the timing from about 1 to 25 seconds with the values shown and a supply voltage of 13.8v. When the power is removed, the diode will feed the charge from the capacitor back to the supply where the OP-Amp supply current will discharge it safely.

This circuit can be used to form a timer from about 1mS (0.001 second) to many minutes, or even hours. Modern SEDs (Solid Electrolytic Devices) can have a capacity of many Farads, which will allow a timing period of a few million seconds. 1.5 farads = 1,000,000 seconds or about 11 days. 6.8 farads + a 10M resistor will give you about six months. You may need to use an FET OP-Amp for accurate timing, but since the reference and the capacitor charging current are both related to the same supply voltage, the time delay is quite independent of supply voltage.

Incidentally, if you reverse the '+' and '-' input terminals of the OP-Amp then the output of the Amp will give a HIGH output (+ve volts, up to 20mA) for the duration of the timer, then will switch to 0-volts. This can be used as the supply voltage for a repeater 'Tone-Burst' generator or a Citizens Band (CB) 'Roger Beep' generator. One final point, as soon as I post a project I usually get loads of E-mail with questions. Let me answer one of them right now: NO, it is NOT possible to make the timer output go HIGH when the DC input power is removed!

This circuit is presented as an idea, therefore no PCB, no kit, no-nothing! but I hope you have fun with it. If I ever have a couple of hours spare with nothing to do then I may just build it it, prove it and photograph it, but don't hold your breath. Very best regards from Harry - SM0VPO

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