Thanks, will experiment with it as suggested.

@hw_user I'm interested in implementing something similar to this (pushing a remote control button for about a second). What parts did you end up using?

Moderator edit: New thread created from this.

 

Depending on what voltage you choose and which coil you get; this circuit will energize the relay for only as long as it takes for the cap to charge up triggering the relay until charged. The resistor needs to be large enough so that the coil doesn't remain active once triggered but small enough to allow somewhat rapid retriggering of the relay.

View attachment 281275
When you push and hold the button the cap will begin to charge, drawing a fair amount of current. That current (and proper voltage) will close the relay until the cap charges enough to drop the current below the Hold Threshold of the coil. If you're using an electronic signal to trigger the relay then that signal source needs to be able to supply sufficient current to drive the relay. The diode becomes even more important because it prevents back feeding of a high voltage spike which can damage your electronics. The relay contacts need to be rated to handle the amount and type of voltage AND current.

@Tonyr1084 Would it be possible to switch out the relay for a solenoid to simulate a physical button push? I'm trying to flip a switch up that simulates a one-second button push on a remote control, then flip the switch back down again to simulate another. It might be possible to hack the remote and use the relay method, but that might not be an option in this case.

Also apologies if reanimating this thread after 6+ months of inactivity is against the rules. If I need to start a new one I'll be happy to do that.

 

Hi Cinematic6436,

Here's what you need.


In one position of the SPDT switch, the relay pulses as the capacitor charges through its coil.

In the other position the relay pulses again as the capacitor discharges through its coil.

The pulse width is determined by the time constant (relay coil resistance * capacitance).

Nandu.

 

One additional thing is that with using a capacitor to drive a relay coil, either series or parallel, the change in relay voltage will be slow enough that no diode protection will be needed.

 

One additional thing is that with using a capacitor to drive a relay coil, either series or parallel, the change in relay voltage will be slow enough that no diode protection will be needed.

Thank you -- that thought crossed my mind because of the earlier exchange.

 

Hi Cinematic6436,

Here's what you need.

View attachment 297430
In one position of the SPDT switch, the relay pulses as the capacitor charges through its coil.

In the other position the relay pulses again as the capacitor discharges through its coil.

The pulse width is determined by the time constant (relay resistance * capacitance).

Nandu.

Thank you for the quick response, Nandu! I found an SRD-S-112D relay (12A 125VAC/7A 250V, if it matters) left over from a previous project. The coil voltage is 12 volts. Do you know what voltage/uF capacitor I'd need? The one-second specification isn't critical -- it could be 1-5 seconds or even 1-10 if necessary.

Brian

 

I have one of those relays. The coil is 400 ohms. A 2000uf cap gives appx a 1 second pulse at 12 volts.

 

If the circuit being switched is normally biased at 5 or 12 volts DC (sourcing load) then you can also probably simulate the push button with a capacitor of a much lower capacitance, ten microfarads or less. That has worked very well for other folks in the past. You will need to put a high value resistor ( 1 megohm )across the capacitor to discharge it. That will work because the load resistance of the circuit is much higher than the 400 ohm relay coil.

 

Thank you for the quick response, Nandu! I found an SRD-S-112D relay (12A 125VAC/7A 250V, if it matters) left over from a previous project. The coil voltage is 12 volts. Do you know what voltage/uF capacitor I'd need? The one-second specification isn't critical -- it could be 1-5 seconds or even 1-10 if necessary.

Brian

Anytime, Brian!

Time constant = μF * MΩ

With the relay coil resistance being 400 Ω, a 2000 μF capacitor will give a 0.8 s pulse ( 2000 * 0.0004 = 0.8 s).

You may choose the required value of the capacitor by trial.

Nandu.

 

Anytime, Brian!

Time constant = μF * MΩ

With the relay coil resistance being 400 Ω, a 2000 μF capacitor will give a 0.8 s pulse ( 2000 * 0.0004 = 0.8 s).

You may choose the required value of the capacitor by trial.

Nandu.

@sghioto & @vu2nan

Thanks! Does voltage matter? I'm seeing 16v and 50v -- will anything over 12v be acceptable?

 

@sghioto & @vu2nan

Thanks! Does voltage matter? I'm seeing 16v and 50v -- will anything over 12v be acceptable?

So sorry, Brian, I missed mentioning the capacitor voltage rating. 16 V or 24 V should be okay.

Nandu.

 

Um, let’s take a step back. The relay is controlled via the internet, which means it is controlled by a microcontroller or other processor. Why are we talking about capacitors on the relay when the on time of the signal is completely controllable via software?

 

Um, let’s take a step back. The relay is controlled via the internet, which means it is controlled by a microcontroller or other processor. Why are we talking about capacitors on the relay when the on time of the signal is completely controllable via software?

That was from the original TS. The topic picked up again at post #14, not sure if this still involves the internet or software.

 

I should also mention that the 1 second time was measure not just calculated. Because the dropout voltage for the relay is lower then the pull-in voltage. Increasing the supply voltage will also extend the time.

 

Didn't realize the thread had been hijacked.

 

Didn't realize the thread had been hijacked.

All subsequent posts moved to this new thread.

 

Um, let’s take a step back. The relay is controlled via the internet, which means it is controlled by a microcontroller or other processor. Why are we talking about capacitors on the relay when the on time of the signal is completely controllable via software?

I actually don't need the wifi/internet elements, just a simulated button press (I intend to use a solenoid with the relay). Someone else started this thread, and I, uh, hijacked it.

 

So sorry, Brian, I missed mentioning the capacitor voltage rating. 16 V or 24 V should be okay.

Nandu.

Hello again --

Just a quick question. I found another relay lying around -- a Huike HK3FF-DC6V-SHG. Since it'll only be powered for a moment, would it be safe to swap the original 12 volt Sanyou SRD-S-112D relay we discussed with this 6 volt one, or would that be unsafe or unreliable in the long run?

If not, I can just buy another Sanyou.

 

That relay has a coil resistance of 100 ohms so would need a much larger cap to get a 1 second pulse. Would be safer installing a 47 ohm resistor in series with the relay coil.

 

That relay has a coil resistance of 100 ohms so would need a much larger cap to get a 1 second pulse. Would be safer installing a 47 ohm resistor in series with the relay coil.

Thanks for the suggestion. It doesn't need to be a full second -- it just needs to be long enough to trigger a solenoid to push the button on a remote control. I'll try it out tomorrow and see if it works. Hopefully I have that type of resistor in stock...