Shorten relay trigger with capacitor?

Thread Starter

Valhallan

Joined Apr 10, 2021
10
So I have a Hik-Vision DS-KB8113-IME1 IP door bell that has a "door trigger" relay that you can fire from the app.

I hooked this up to my old school ET 500 Gate motor as it has a wire in trigger.

The trouble is the relay on the doorbell is set to its lowest "open time/ trigger time" of 1 sec. This is unfortunately to slow for my ET 500 and is resulting in a double trigger event, i.e. the gate stops half way. I can reproduce the issue with the standard remote by holding in the button too long. As far as I can tell there is no way to change the timing on the ET 500 and I have tried all configurations on the doorbell. A trigger signal longer than I'm guessing 0.5sec triggers a new event on the gate motor.

Solution
I need to shorten the duration of the trigger signal. Reading a thread on this site I see it may be possible to use a capacitor with a bleeder resistor to solve this.

As I understand it, the capacitor would in this case ideally spend the first 0.8 sec charging and only fire at the end. The bleeder will discharge the capacitor between trigger events.

Question
If this is a viable solution (I'm a total noob at circuitry) and as the trigger is a 5V signal from the gate, what size/type capacitor and bleeder do I need?


Thanks
 

Attachments

LesJones

Joined Jan 8, 2017
3,498
It is feasible but choosing the correct value of capacitor is probably going to trial and error unless you know the current through the input connections on the gate controller. You could measure this or if you can post the schematic of the gate controller we could probably work it out. Your understanding of how the capacitor works is not quite right. The capacitor is discharged before the button is pressed. (Or the contacts close in the doorbell trigger.) So initially the capacitor looks like a short circuit but as the capacitor charges the current decreases. When the current gets low enough the gate controller considers it's input terminals as open circuit. The resistor in parallel with the capacitor must be high enough resistance to be considered an open circuit by the gate controller.

Les.
 

Thread Starter

Valhallan

Joined Apr 10, 2021
10
It is feasible but choosing the correct value of capacitor is probably going to trial and error unless you know the current through the input connections on the gate controller. You could measure this or if you can post the schematic of the gate controller we could probably work it out. Your understanding of how the capacitor works is not quite right. The capacitor is discharged before the button is pressed. (Or the contacts close in the doorbell trigger.) So initially the capacitor looks like a short circuit but as the capacitor charges the current decreases. When the current gets low enough the gate controller considers it's input terminals as open circuit. The resistor in parallel with the capacitor must be high enough resistance to be considered an open circuit by the gate controller.

Les.
So based on that the arrangement of the circuit should be as follows?? Obviously I use random values here for the resistor/capacitor. (See image)

Forgive me, I'm a total novice here :p
 

Attachments

DickCappels

Joined Aug 21, 2008
7,746
He probably meant neither - the capacitor should be in series with the relay coil, not the relay switch. The 100 ohm resistor looks very small compared to the level of the currents available and the 5V supply, and you will confirm this when you build it, but the 1 uF probably won't give you much contact closure time.

How many volts does the relay need to fire and what is the coil current (8 ma?)?
 

Thread Starter

Valhallan

Joined Apr 10, 2021
10
He probably meant neither - the capacitor should be in series with the relay coil, not the relay switch. The 100 ohm resistor looks very small compared to the level of the currents available and the 5V supply, and you will confirm this when you build it, but the 1 uF probably won't give you much contact closure time.

How many volts does the relay need to fire and what is the coil current (8 ma?)?
Yeah the values for the resitor and capacitor in the image where just place holders really.

So the doorbell/relay coil isn't accessible to me...its part of a closed proprietary system.

I'm trying to workout if there is anything I can add between the gate trigger and the doorbell relay that will give the desired "shortening" of the trigger signal
 

Thread Starter

Valhallan

Joined Apr 10, 2021
10
So from what I can see the gate is sending a constant 5v open circuit. When the gate detects the the circuit closes it opens the gate.

I have wired that into the doorbell which powers its own relay. That relay closes the gates circuit for 1sec.

I need to shorten the signal period that the gate is detecting without modifying the relay or the signal source....I can only add co.ponents between the two.

Hope that makes sense o_O
 

LesJones

Joined Jan 8, 2017
3,498
The circuit in post #5 is the way it should be connected but I suggest starting with about a 470 uF capacitor and a 10 K ohm resistor. (The + end of the capacitor to the gate input.) I think this will give about a 300 mS pulse. with that value of resistor you will probably have to wait for about 10 seconds in between pulses for the capacitor to discharge. If you lower the value of resistor it will shorten the time to discharge the capacitor but if it is too low the gate input will see its input as a low level as long as the relay contacts are closed. If you can't wait as long as it takes to discharge the capacitor we will have to design a more complex circuit. I suggest using a push button switch in place of the relay contacts for testing.

Edit. I've just seen your answer to Dick's in your post #12 I have not understood what you are doing. I thought the remote door bell relay was required to trigger the gate controller. If the signal is going from left to right it will not work as applying a voltage to a set of contacts does nothing.

Les.
 
Last edited:

Thread Starter

Valhallan

Joined Apr 10, 2021
10
The circuit in post #5 is the way it should be connected but I suggest starting with about a 470 uF capacitor and a 10 K ohm resistor. (The + end of the capacitor to the gate input.) I think this will give about a 300 mS pulse. with that value of resistor you will probably have to wait for about 10 seconds in between pulses for the capacitor to discharge. If you lower the value of resistor it will shorten the time to discharge the capacitor but if it is too low the gate input will see its input as a low level as long as the relay contacts are closed. If you can't wait as long as it takes to discharge the capacitor we will have to design a more complex circuit. I suggest using a push button switch in place of the relay contacts for testing.

Les.
Fantastic...thanks so much...I will give it a go
 

Thread Starter

Valhallan

Joined Apr 10, 2021
10
Does that doorbell camera support the ONVIF standard, or does it only work with the HIK cloud service?

Also if the above doesn't work, I think this might do the trick (LTC6993):

https://www.digikey.com/en/product-highlight/l/linear-tech/ltc6993-timerblox

There are even development breakout boards for them:

https://www.digikey.com/catalog/en/...93-3-and-ltc6993-4-demonstration-boards/42925
Thanks...yeah it doesn't havethe ONVIF standard no.

Will check that out
 

Thread Starter

Valhallan

Joined Apr 10, 2021
10
Some feed back...

Big thanks to
LesJones, this worked like a bomb first time :cool:

I hooked it up in abit of a janky way but I will neaten it up later. I used a potentiometer for the resistor incase I needed to adjust it. But the travel time of the gate matched the discharge window perfectly.
 

Attachments

Top