Looking for a Circuit that can Detect an Energized AC Relay Coil

Thread Starter

jsthomps

Joined Mar 30, 2010
31
Hi all,
Newbie here. I'm into restoring electromechanical arcade games. I would like to design a sensor/circuit that can detect when a AC relay coil is energised. This circuit would output a TTL HIGH when the coil is on and a TTL LOW when it is not on. I have tried using a reed switch but when the coil is energised the reed is switching at 120Hz. The AC relays in question are show below. In this game the relay coils are rated for 50VAC.
Thanks in advance.

Screenshot 019.JPG
 

Bernard

Joined Aug 7, 2008
5,686
I do not see any NO contacts available. Is one side of 50 V AC tied to common, - ground?
Might not be easy but at one time long ago I needed a low DC V derived from a 120 V AC coil. Was able to add some turns around coil, rectified & filtered for the required 9 V @ 5 mA.
 

k7elp60

Joined Nov 4, 2008
562
I am not sure if I understand exactly what you want. Looking the the relays in the picture you posted I could tell when a relay is energized as the wiper contacts would be pulled down on the lower set of contacts. Do you want another circuit to check the operation of the relays? It is possible to build a circuit that would sense the coil of the relay being energized and supplying a TTL signal. Please be more descriptive in what you would like
 

AnalogKid

Joined Aug 1, 2013
8,692
How fast do you need the TTL signal to be? That is, how long a detection delay can you stand? The idea above of capacitor/bridge/optoisolator/missing pulse detector/output driver is a very common approach, but the circuit output response time would be between 1 and 5 ms for turn on and around 50 ms for turn off. If any of the relays are switching DC to anything, detecting that change would be faster and much more consistant.

ak
 

Thread Starter

jsthomps

Joined Mar 30, 2010
31
Hi everyone,
Thanks for all the responses sent so far. I will try to explain the situation more completely. I like restoring old EM arcade games but I also to to mod and enhance them as well. For example many games have to sound effects except for many some bells or chimes. The circuit that I want to build would be totally independent on the game electrics (no mods whatsoever). This is to conserve the integrity of these historic devices. With that said the circuit would include some type of sensor that would be placed in close proximity to an AC relay coil. The sensor would send a TTL HIGH to a micro-controller when the coil is energized. The MC would then trigger the playing of a sound effect. For example if the "trigger" relay was energized the circuit would play a torpedo launching sound effect.
Screenshot 022.JPG
 

Alec_t

Joined Sep 17, 2013
11,744
To be totally independedent of the game electrics I'd go with Bernard's suggestion of adding some turns around the coil to pick off a signal. Looks as though there'd be room for quite a few turns of enamelled wire.
 

Thread Starter

jsthomps

Joined Mar 30, 2010
31
To be totally independedent of the game electrics I'd go with Bernard's suggestion of adding some turns around the coil to pick off a signal. Looks as though there'd be room for quite a few turns of enamelled wire.
That sounds good but this noob needs to know how to convert the inducted current from the coil in a solid ON/OFF TTL output.

Also I failed to mention before that it would be useful to be able to measure the duration of the coil "ON" time. Most relays in these games energize for a second or two but some pulse for duration of around 100ms
 

BR-549

Joined Sep 22, 2013
4,938
It might be worth mentioning that most I/O ports can use analog signals now. And there are several ways to do this. Are you still requiring TTL? Were you planning on scanning the ports....or using interrupts?
 

Thread Starter

jsthomps

Joined Mar 30, 2010
31
It might be worth mentioning that most I/O ports can use analog signals now. And there are several ways to do this. Are you still requiring TTL? Were you planning on scanning the ports....or using interrupts?
I was hoping for a TTL solution if possible ... then go for an analog solution. Have not decided on port scanning vs interupts at this stage. signal timing is not a big issue with the slow speeds of the inputs
 

Alec_t

Joined Sep 17, 2013
11,744
how to convert the inducted current from the coil in a solid ON/OFF TTL output.
If you have enough turns in the coil you could directly rectify the coil output to get a waveform of sufficient amplitude to reach the logic switching threshold of the micro. Counting rising/falling edges of the waveform would give you the duration of energisation.
 

Thread Starter

jsthomps

Joined Mar 30, 2010
31
Thanks everyone for all the cool suggestions however my circuit design skills are, well, almost non-existent. A circuit design sketch of any or all suggestions would be greatly appreciated.
 

Thread Starter

jsthomps

Joined Mar 30, 2010
31
I do not see any NO contacts available. Is one side of 50 V AC tied to common, - ground?
Might not be easy but at one time long ago I needed a low DC V derived from a 120 V AC coil. Was able to add some turns around coil, rectified & filtered for the required 9 V @ 5 mA.
Sounds good ... any suggestions as to how many turns for a 50VAC coil?
 

Alec_t

Joined Sep 17, 2013
11,744
If you have access to a 'scope you could wind a handful of turns and check the induced voltage. That would enable you to calculate the number needed to get, say, 3-4V RMS.
 

R!f@@

Joined Apr 2, 2009
9,734
I am on vacation and I am using a laptop. The idea just popped in to my head, when you said you used reeds :(
But I believe our 555 guru can rip up a circuit quite fast. Infact there might already be a monostable with adjustable time delay in one of his threads.

If I were at home I could have drawn you the circuit and simulated it but with this laptop, sorry :oops:

Question is, can you do with a little delay. Time delay regarding the monostable pulse

PS. Link added
 
Last edited:

crutschow

Joined Mar 14, 2008
25,660
But I believe our 555 guru can rip up a circuit quite fast. Infact there might already be a monostable with adjustable time delay in one of his threads.
That may smooth out the reed switch pulses, but the reed switch won't last long switching at double the line frequency. :eek:
 
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