Pulse output from IP relay trigger on/off

Thread Starter

sparkie007

Joined Apr 26, 2019
4
I have a network / IP relay (SPST) that triggers on receipt of a specific event (in this case audio traffic on a specific RTP port). The relay is simple in operation, closing during traffic (which could range from a few secs to perhaps a minute), and opening when no traffic. I need to connect this relay output to a device that requires a momentary (0.5s) short (a pulse if you like) to switch it on and the same to switch it off. So, my question is how can I couple the relay output (which will be on constantly during receipt of RTP traffic) to switch on/off the device?

Essentially I need a momentary 'short' during start/receipt of traffic (to turn on the device) and another momentary 'short' at the end/finish of the traffic.... but the relay is constantly active during traffic.

Others have asked similar questions and it seems a 555 circuit of some description may provide an answer..... but I just cant find something to match my application.

Any thoughts?
 

KeithWalker

Joined Jul 10, 2017
522
I have a network / IP relay (SPST) that triggers on receipt of a specific event (in this case audio traffic on a specific RTP port). The relay is simple in operation, closing during traffic (which could range from a few secs to perhaps a minute), and opening when no traffic. I need to connect this relay output to a device that requires a momentary (0.5s) short (a pulse if you like) to switch it on and the same to switch it off. So, my question is how can I couple the relay output (which will be on constantly during receipt of RTP traffic) to switch on/off the device?

Essentially I need a momentary 'short' during start/receipt of traffic (to turn on the device) and another momentary 'short' at the end/finish of the traffic.... but the relay is constantly active during traffic.

Others have asked similar questions and it seems a 555 circuit of some description may provide an answer..... but I just cant find something to match my application.

Any thoughts?
What contacts on the relay do you have available to you?
 

Thread Starter

sparkie007

Joined Apr 26, 2019
4
It's a SPST v/free relay output - both contacts are available.

The fact the relay is triggered by RTP IP traffic is I guess irrelevant.... Simplistically, I have a SPST switch output that I need to use to generate a momentary short when energised and de-energised.

Thanks.
 

crutschow

Joined Mar 14, 2008
23,758
What power do you have available?
What pulse amplitude do you need and what is the load impedance?

An XOR gate, such as the CD4070 can be easily configured to generate a pulse on both the rising edge and the falling edge of the signal.
A wallwort can be used to power the circuit if needed.

Below is a typical circuit showing an output pulse on both the leading and trailing edge of the input pulse:
For your application, some additional debounce circuitry would also be needed.

upload_2019-4-26_10-12-10.png
 
Last edited:

eetech00

Joined Jun 8, 2013
1,738
Another consideration is what duration of relay “on” time qualifies as “on”....same goes for “off” (not referring to contact bounce although it’s needed).

eT
 

Thread Starter

sparkie007

Joined Apr 26, 2019
4
My application uses a trigger from a volt free SPST relay - I used the word 'pulse' in the context of a 'short' being applied for a small period of time when the relay switches on and off. In that regard, I don't need any specific 'signal' or amplitude of voltage. I guess a better description post title would have been "momentary relay trigger output on state change - on/off".

With regard relay bounce, I am concerned about that and will look to apply some sort of debounce cct to help/eliminate.

crutschow: That XOR gate, CD4070 cct looks interesting. I guess I could pass the output via a transistor/mosfet to drive another SPST relay which in turn will hopefully give me the momentary short I need?

Think that will work?
 

crutschow

Joined Mar 14, 2008
23,758
I guess I could pass the output via a transistor/mosfet to drive another SPST relay which in turn will hopefully give me the momentary short I need?

Think that will work?
Yes, it should.
Do you have any power available?

Below is the LTspice simulation of the circuit with input signal from a relay, and driving a relay output.
Four Schmitt-trigger NAND gates (one IC package) are used to implement the XOR function, to avoid any oscillations during the slow RC rise and fall transition times of the gate input signals.
R1C1 determines the output pulse width.
R2C2 is the delay for the contact debounce function.
It will work on any DC voltage from 5V to 15V as long as the output relay has the correct coil voltage.

Does that do what you want?

upload_2019-4-26_19-53-25.png
 
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