Timer circuit that can ignore a persistent trigger current.

I want to have a low voltage DC lamp illuminate briefly to confirm to me (from the exercise studio at the bottom of my garden) that the studio heater has been switched on – I’ll have used an infra-red remote control device to switch on the power in the first place from the comfort of my house (so I don’t have to brave a cold trot to the bottom of the garden and back just to switch it on). It’s simply that I need to see a confirmatory signal coming from the exercise studio (i.e. the low voltage lamp) so I know the heating has actually been turned on.

What I’ve done so far.

I have studied various circuits involving the NE555 timer in an attempt to solve the problem. Unfortunately these only seem to work when the signal current that initiates the 555 circuit is a brief pulse, like from a push to make switch, but not when there’s a constant signal current (as happens when the power to the heaters remains on and persistently triggers the 555 circuit).

At the moment when the mains power comes on in the studio (i.e. I’ve pressed the remote control from within my house), it is that mains power that activates a mains-rated 240VAC relay coil which, in turn, simply completes the 555 timer circuit. This, in turn, illuminates the low-volt lamp and lets me know the heating is on.

Key problem

The trouble is that it is the continuation of the power to the mains-rated 240VAC relay coil that, of course, persistently triggers the 555 timer circuit with the net result that the low-volt lamp remains on regardless of the efforts of the 555 circuit to switch it off!

Hence, do you have a diagram for a low-voltage DC circuit that will trigger the illumination of the low voltage lamp for a few seconds and then, by ignoring the persistent trigger from the mains-rated 240VAC relay coil, switch the lamp off even though the mains current continues to trigger the 555 circuit by keeping the 240VAC relay in the closed (on) position?

I hope that I'm being clear enough in my explanation?!


Use the 555 in Monostable mode (one shot). When connected in this manor it gets a signal and runs once, it does not work again until you send it another signal. I can go more into it if you want or someone else may want to give you an even better idea.

I will see what I can come up with for a design at work tonight. Could you give me some more info such as what voltage will be the input to the 555 circuit, I am assuming it wont be the 240 or at least hope not. I also see you are using a relay to send voltage to the 555 circuit, have you considered just using an off delay time delay relay? That could eliminate the 555 circuit all together as well as your constant signal issue. The off delay timer would just time for XX sec/min after being energized at which point the contacts would go back to their normal state. I used to work a lot with relay logic on higher voltage machinery such as 120/240/277...... so I am pretty knowledgeable about the relays available for such a circuit. If you could please reply before 10 pm EST - east coast U.S. time and I will work out something at work tonight.


Hi George, and thank you for your most helpful reply.
I'm not fussy about what particular components I use, I'd like to choose those that give the simplest and most economic solution.
I'm most impressed with your suggestion of the time delay relay solution. This looks like a good way forward.
With regard to the voltage input to the chosen device, I prefer to work with low current (i.e. less than 200 milliamps) at 12Vdc where possible and, when 240 volt mains power is being controlled, I usually use a robust 30 amp relay that has, of course, a 12 volt dc coil.
Basically for my needs the device, say a time delay relay, would only need to operate within a 12 volt dc environment, if that makes sense - I would avoid applying 240 volt ac mains power to any such item!
In fact there are 2 devices I would like to put together:
  • The first is the timer circuit I talked about above for which I require the lamp to come on for only a few seconds when the heating circuit is switched on.
  • The other is a device that must do the same job as the first one, but which needs to come on for about 1 hour instead of a few seconds.
Would you have any suggestions as to specific time delay relays that I might consider using for these purposes?
I live in the UK, so if there are suppliers here that offer similar spec items to those you'd chose in Fleming Island, this would be much appreciated.
Many thanks for your help.
Tim, simply put the input of the circuit through a capacitor. Capacitors block DC and will let the 555 switch off after a few seconds. Start with a 1uf electrolytic maybe?

Tim, simply put the input of the circuit through a capacitor. Capacitors block DC and will let the 555 switch off after a few seconds. Start with a 1uf electrolytic maybe?

Wont the cap totally block the dc from ever letting the 555 even get the signal to start it the first cycle?
No it will let a pulse (ie 0 to 12v) through at first and as it charges up (very quickly for this size) it will go open circuit.
The circuit may need a 10k resistor from input to ground so that the input is held low after the pulse
No, on second thoughts use a 100nf ceramic
Thank you Fleck, you've solved the first of my timer issues (please see an earlier post) and apologies for possibly poor use of terminology due to inexperience.
I've put the input (pin 2) through a 100nF ceramic (at least, in a virtual circuit on my PC).
This changes the behaviour of the 555 timer (which is in a monostable mode circuit, I think) such that I can use the "sinking" current that occurs when the circuit is powered on to drive a relay that connects the signal lamp circuit. This "sinking" current switches itself off after a few seconds, and remains off, in spite of the persistent trigger current. Added to this, there is virtually no residual current flow after that and the circuit neatly resets itself when everything is powered off, as will happen at the end of my exercise session.

So, nice one, thank you; problem 1 sorted (provided I can master the required soldering techniques , of course)!

Problem two still exists though, i.e. I want the device (e.g. a 12 volt DC lamp) to remain on for approximately 1 hour (this is for a different project as explained above) and, as far as I'm aware, a 555 timer circuit is not suitable for timing anything over 10 minutes?
George has suggested a time delay relay which looks really good and I may well need to purchase such a device if I can't construct my own version.
So, I wondered if a circuit diagram existed that was sufficiently simple for a relative newbie, like me, to build?
The device will not need to be adjustable, it "simply" needs to provide approx 1 hour of power to a 12 volt DC lamp, then power off in spite of a persistent trigger current and reset itself when the entire circuit is turned off (as in problem 1 above).
Thanks for your help.
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A binary counter is good for longer periods.
It would be pulsed by a 555 and the various outputs would go high after a certain number of clock cycles. Look for a 14 stage binary counter, output 14 would go high lastly. If you string them together you can quickly get VERY long time spans. 1 would do for your circuit though.
They are easy to build & only need a couple of components round the IC
If the 1 hour is approximate just consider uprating the resistor and capacitor on the monostable. Would be simpler, but never tried it myself
That's great, thank you. I shall enjoy researching the binary counter though, first, I shall need to review the circuit that makes the 555 deliver pulses when I get back from work.

Have a look at this. The transistor might need changing to a bigger one depending on the relay coil current.
Also the 100k resistor is just a guess.
I can't remember what to do with the reset pin but it will be easy to Google and find out.
Hi George, what would be the best type (i.e. electrolytic or ceramic) and spec for the capacitor to connect the reset pin to ground?
I'll try this circuit out on my PC but I notice that the software I'm using (Circuit Wizard, http://www.new-wave-concepts.com/) supplies a component listed as a 4020B 14-stage binary ripple counter which looks like this on my screen.
4020B ripple counter.PNG

Do you think this should match the 4020 suggested by Fleck? Because it has a "pin 11" and an "R" (reset?); also it doesn't have "pin 16" except for the unlabelled pin at the bottom left of the image.
Maybe I'm being a bit picky here due to inexperience. It's no big deal. If I've asked an impossible-to-answer question I can either experiment with the version supplied on my PC or just build the circuit anyway and see what happens (using, of course, a low voltage mock up circuit first, before applying it to control mains power).
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Ceramic 100nf

I was thinking you don't want the counter to reset when it gets to 11111111111111 so I couldn't remember what to do with the reset pin ie leave it high or low. It'll all be on the data sheet for the IC anyway
Also put a ceramic 100nf across the supply, it's good practice to do as any noise or AC spikes or pulses will be shorted to ground. Be kind to your chips :)
That counter will be the same I'm sure.
I don't use computer simulators so I don't know why all the pins aren't present.

Bear in mind too that the outputs labeled Q are not the same as the pin numbers on the chip
That counter will be the same I'm sure.
I don't use computer simulators so I don't know why all the pins aren't present.

Bear in mind too that the outputs labeled Q are not the same as the pin numbers on the chip
That is a very good point Fleck! I use the same sim Tim is using and they don't always have all the pins. I know off the top of my head the 555 timer in that sim does not have a pin 1, I guess they assume you will ground the chip but I'm not sure. I have also found that circuits are not 100% correct as far as how they operate in that simulator. Sometimes it is best just to buy a few chips and play around while looking at the data sheet. I attached the datasheet here. I will be buying a few of these myself in the next week for a project I want to make. If this post is still active at that time I will give more info. I find the best way to learn these things is by just playing around with them, at least for me.

Tim, as for what cap is the best I don't know, Fleck knows his stuff so you should try the Ceramic 100nf he mentioned above.