Need a 60 second timer

Thread Starter

teutontech

Joined Apr 10, 2023
12
Use case:

User presses button, timer turns on for 60 seconds to power my relay, then turns off.

This will be run off of batteries, but I do not want the timer to drain the batteries.

Are there any options available on amazon? All of them I found if you give them power ALL stay on, and do not automatically shut off.
 

crutschow

Joined Mar 14, 2008
34,428
Are there any options available on amazon?
Don't know of any offhand.

Below is the LTspice simulation of a 555 (LM555, NE555 or similar) timer circuit with zero quiescent power (just the very small transistor leakage current which is likely less than the battery self discharge current)
It uses a couple of transistors (Q1 and Q2) to completely remove the circuit power after the timeout.

The pulse on-time is determined by the value of R2 and C1.

What is your battery voltage (V1)?
It can be anywhere from 4.5V to 16V.

At the bottom is the sim of an alternate circuit that uses one CD4093 quad 2-input Schmidt trigger NAND gate package to generate the pulse.
The green trace is the relay coil current.
It requires a few less parts and one fewer transistor, but is has somewhat lower timing accuracy than the 555 circuit, if that's a concern.
The quiescent power for this is also just the very low leakage currents of the IC and the transistor.

Either circuit can be readily assembled on a small Vector-type perf-board.

They do what you want?

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WBahn

Joined Mar 31, 2012
30,055
Use case:

User presses button, timer turns on for 60 seconds to power my relay, then turns off.

This will be run off of batteries, but I do not want the timer to drain the batteries.

Are there any options available on amazon? All of them I found if you give them power ALL stay on, and do not automatically shut off.
If you don't want the timer to drain the batteries, then how do you plan to power the timer? It has to get energy from somewhere in order to work and if that energy comes from the batteries, that will be a drain on them. Do you want the timer to be some kind of mechanical wind-up timer? Those used to be pretty common, but I don't know how hard it would be to find them these days.

What are the voltage and current requirements for your relay?

What price range are you looking for?

Look over these and see if there's something there that would do what you need. If you find one, point it out and then we can discuss less costly alternatives to achieve the same behavior.

https://www.grainger.com/category/e...fety/relays-and-accessories/time-delay-relays
 

ElectricSpidey

Joined Dec 2, 2017
2,779
" If you don't want the timer to drain the batteries, then how do you plan to power the timer? "

Simple, just have the button power up the entire circuit, then use a spare contact on the relay to maintain the power, or if there is no spare contact use a transistor driven off of the 555 output.
 

crutschow

Joined Mar 14, 2008
34,428
But I would probably use a single NMOS on the low side instead of the 2 bjts on the high side.
I tend to avoid switching the low-side of IC's, due to sneak problems.

For example, my simulation showed that as the transistor started to turn off, the capacitances in the circuit caused the OUT voltage to stop going low as the transistor was turning off in sort of a positive feedback connection.
This turned the transistor and the 555 partly off and on, generating oscillations with the 555 never turning completely off.

So you are welcome to try that, and there may be a way around the problem, but I'll just stick to high-side switching.
 
If the exact timing of the "relay on" is not critical, arrange for the push button to charge a capacitor which discharges through a resistor into the base of an NPN transistor with the relay between the collector and positive rail. The transistor will turn off when the base/emitter voltage falls below 0.6V. Use a darlington pair transistor for a lower value of capacitor or longer 'on time'. If the relay is handling high current you'll want the transistor to turn off quickly - a schmitt trigger IC can be used to turn the transistor on/off, in turn driven by the the capacitor discharging through a resistor and zener diode. I think this would work, I expect to be corrected!
 

WBahn

Joined Mar 31, 2012
30,055
I assume then, that you didn't look at the circuits in my post. :rolleyes:
The question was very clearly asked of the TS, not you. So your circuit was irrelevant to the question regarding how the TS is expecting to power the timer if it's not allowed to be a drain on the batteries.

But it sure looks like your circuit powers the timer from the battery. It may only drain the battery while the timer is running, but it is still draining the battery while the timer is running. The TS very specifically said that they didn't want the timer to drain the batteries. Period. So either they really do not want the timer to draw energy from the batteries for some reason, in which case the alternate source of power needs to be identified, or they ARE okay with it drawing SOME power from the batteries, but need to have it be less than some maximum allowed amount, in which case that maximum amount needs to be identified to ensure that the approach used does not exceed that.
 

WBahn

Joined Mar 31, 2012
30,055
" If you don't want the timer to drain the batteries, then how do you plan to power the timer? "

Simple, just have the button power up the entire circuit, then use a spare contact on the relay to maintain the power, or if there is no spare contact use a transistor driven off of the 555 output.
Not simple if there is no power source other than the batteries. If the relay is being powered by some other source, or is switching some other source, then all kinds of games can be played. But if the only source of power is the batteries, then the timer is going to have to get it's energy from the batteries. You can do things to minimize the drain, but the timer WILL drain energy from the batteries unless it is powered by something other than the batteries.
 

crutschow

Joined Mar 14, 2008
34,428
But it sure looks like your circuit powers the timer from the battery
Of course that's obvious, otherwise we are talking about the impossible, which you seem fixated on due to the imprecise wording by the TS.

It would seem the apparent meaning was that the TS meant the timer wouldn't be powered when it was not operating, but you took the literal meaning, which makes little sense.
It takes power to operate the relay, which apparently comes from the battery, so the logical assumption is that a tiny amount of that power could be used for the timer when it is timing and powering the relay.

So I believe your response to the TS was "irrelevant".
 
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