One-shot relay with persistent supply

Thread Starter

Mark James

Joined Mar 10, 2018
14
Hi all,

I have tried to search for the answer to my question, but don't know the correct terminology so it has been difficult to determine what to search for.

I am modifying an old games console to contain an FPGA rebuild of the original console circuit board. The board requires 5v and has a micro-USB connector for the power, and I want to use the original power switch from the housing to switch it on and off. I have already wired up the original case's power switch to a micro USB cable so can use the original power switch to make and break the supply of power to the board.

The problem is that the board has a power button of its own which must be pressed in order to get it to power on. There is no way of getting it to power on automatically when current is provided. The power switch is non-latching so only needs to be pressed momentarily.

My plan is to power a relay circuit connected in parallel to the 5v supply coming into the case. This circuit should wait for a fraction of a second to make sure the console's main board is powered, then close the relay for a fraction of a second, and then open the relay again. The relay should not be triggered again until the console is fully powered off and on again. I can then solder wires from the output of the relay to the contacts for the board's momentary switch to simulate the pressing of the button.

I have found some relay timing circuits online, but because I don't fully understand the nomenclature I'm struggling to understand whether they are the right circuit, or if they are the best way of approaching it.

I'd really appreciate any help in advance!
 

Thread Starter

Mark James

Joined Mar 10, 2018
14
Just a quick follow-up to this which might simplify matters. I've just tested the board by holding its built-in power button down. As long as the power button is pressed after the board has received power, it seems to work perfectly fine if the power button is held continuously. In other words, the power button doesn't seem to need to be released after pressing it.

So, this might simply be a case of delaying the powering of the relay for a fraction of a second to ensure the main board gets power first, then powering the relay and allowing it to remain powered.

I would prefer, ideally, for the relay to power off again after simulating the button press, just in case leaving it powered causes problems down the line. But for now at least, simply delaying the relay would seem to be a possible solution, and probably considerably more straightforward than trying to time a power on and off of the relay.

Any thoughts would be most welcome!
 

Thread Starter

Mark James

Joined Mar 10, 2018
14
I'm wondering whether this little board would suffice:

https://www.ebay.co.uk/itm/252456160636?ViewItem=&item=252456160636

There is a 'delayed off' variant. If I had the power button wired up to the normally closed side, then powering the circuit would result in it opening the normally closed side ("unpressing" the power button). After the time delay, it would switch back to the normally closed side ("pressing" the power button).
 

Thread Starter

Mark James

Joined Mar 10, 2018
14
Sorry for so many posts. I have just found this:

http://amzn.eu/0SgNvWT

It has several modes, among which is "A signal trigger and countdown begins, T1 seconds after, the relay closes T2 seconds, then opens". This would seem to be exactly what I need if I have understood correctly.
 

LesJones

Joined Jan 8, 2017
4,171
Just connecting a capacitor in parallel with the switch may solve the problem. You will have to use trial and error to select a suitable value of capacitor. Try 10uF as a starting point. Check the polarity of the voltage across the switch first so you can connect the capacitor with the correct polarity.

Les.
 

Thread Starter

Mark James

Joined Mar 10, 2018
14
Just connecting a capacitor in parallel with the switch may solve the problem. You will have to use trial and error to select a suitable value of capacitor. Try 10uF as a starting point. Check the polarity of the voltage across the switch first so you can connect the capacitor with the correct polarity.

Les.
Thank you Les, this is very helpful. So, would this basically involve breaking the 5v supply to the relay and bridging it with the capacitor? Also, and this probably a very basic question, but once a capacitor has filled and discharged, does it then provide continuous flow of current, or will it continually fill and discharge? Because if it is the latter, then this would cause the console to repeatedly power on and off (i.e. it would simulate repeatedly pressing the power button).
 

Thread Starter

Mark James

Joined Mar 10, 2018
14
Ah, I've just read up on capacitors and it seems I've misunderstood. It will only discharge when the capacitor is switched to something which places load on it, right?

So, breaking the 5v supply to the relay with a capacitor will slow down the rate of flow of current powering the relay and delay its triggering, which is what I need. If I've understood correctly, once the capacitor is charged, current will stop flowing. When the circuit is powered off (the console is switched off), there will be no load on the capacitor, but also no ongoing supply. So presumably this means the capacitor will discharge slowly. Could this cause a problem if, for example, the console was powered rapidly of and on again?
 

LesJones

Joined Jan 8, 2017
4,171
No Just connect the capacitor in parallel with the push button. When power is first applied the capacitor will be dischaged so it wall behave like a partial short across the push button the current that would flow through the push button charges the capacitor so when the capacitor is charged no current is flowing. So now the capacitor behaves as an open circuit. For this to work the circuit must work with a slow transition from current flowing to current not flowing. The push button itself would give fast transition so the capacitor idea may not work.

Les.
 

Thread Starter

Mark James

Joined Mar 10, 2018
14
No Just connect the capacitor in parallel with the push button. When power is first applied the capacitor will be dischaged so it wall behave like a partial short across the push button the current that would flow through the push button charges the capacitor so when the capacitor is charged no current is flowing. So now the capacitor behaves as an open circuit. For this to work the circuit must work with a slow transition from current flowing to current not flowing. The push button itself would give fast transition so the capacitor idea may not work.

Les.
Ahh I see. I don’t think that would work, as the push button has to be pressed after the board has been powered. If the relay was powered too soon, it would precede the board being powered.

Did the circuit boards I linked above look like they could work? They’re cheap and if they would do the trick then it would seem a simple solution.
 

LesJones

Joined Jan 8, 2017
4,171
I now realise I misread your post #2 . I had thought you said if you held the button down then applied power then released the button the unit switched on. Sorry for my mistake.

Les.
 

Thread Starter

Mark James

Joined Mar 10, 2018
14
I now realise I misread your post #2 . I had thought you said if you held the button down then applied power then released the button the unit switched on. Sorry for my mistake.

Les.
Ah, no, the power button must be pressed after power is applied. If it could be held down constantly then I'd just short the switch out (or physically depress it constantly).

Were you able to look at the small boards I linked above? I think they'll solve the problem, but I want to be sure before ordering.
 

LesJones

Joined Jan 8, 2017
4,171
I don't think the board in the link will do what you want. It only seems to do one delay. You need two delays. The first to wait until the poweris fully on, the second to time the pulse. Here is a suggested way of doing what you want. It should delay about one second after the power is applied then pulse the relay on for about one second.
110318.jpg
Connect all the unused inputs of the 40106 to either 0 volts or +5 volts
If you measure the voltage between the negative rail and both sides of the push button (With the button NOT pressed.) it may be possible to remove the need for a relay.

Les.
 

Thread Starter

Mark James

Joined Mar 10, 2018
14
I don't think the board in the link will do what you want. It only seems to do one delay. You need two delays. The first to wait until the poweris fully on, the second to time the pulse. Here is a suggested way of doing what you want. It should delay about one second after the power is applied then pulse the relay on for about one second.
View attachment 148073
Connect all the unused inputs of the 40106 to either 0 volts or +5 volts
If you measure the voltage between the negative rail and both sides of the push button (With the button NOT pressed.) it may be possible to remove the need for a relay.

Les.
Thanks man! The circuit diagram you drew is great! On the subject of the circuit boards I linked, timed power off isn't necessary. Pressing and holding the power button works fine, as long as the press happens after the main board is powered. Would those boards work for that? I'm able to solder and happy to make up a circuit if I know what to build, but those small boards are only a couple of £ to buy which is probably what I'd end up spending on components to make something myself. If I know a pre-made solution would work then I'd rather do that TBH.
 

LesJones

Joined Jan 8, 2017
4,171
I've just had a closer look at the link to "See more product details" from your link and it look like the P-2 mode may do what you want as it gives the two delays you need.

Les.
 

Thread Starter

Mark James

Joined Mar 10, 2018
14
I've just had a closer look at the link to "See more product details" from your link and it look like the P-2 mode may do what you want as it gives the two delays you need.

Les.
Thanks Les, I thought so. I'm going to order one now and will report back. Thanks again for all your help!
 
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