Warning Light

[WildWest]

Hi All,

I need a simple circuit that will light an LED when contacts of a micro switch open. The switch is in series with a gas valve which operates on 24vac. In the "normal" operating state, the switch is closed and the gas valve is allowed to operate. When air flow is low, the switch opens taking power away from the gas valve which will then shut off. The switch only has two connections and it has to be as simple as possible. I want the LED to come on when the switch opens to let operators know the heat is off.

Any ideas on a simple circuit would be apprecated!

Thanks in advance,

Robert

[audioguru]

Hi Robert,
Welcome to our forum.
I was going to tell you where to stick the LED (he, he) and how to wire it, then realized that you shouldn't tamper with gas controls, it's dangerous and should be done by a certified gas fitter.
My idea would be very simple but might make the gas valve stay open when it should shut off:
Ka-Boom!
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[WildWest]

Thanks for the response. In further checking the switch is not actually in series with the gas valve, it is an input to the control logic PCB. As a facility / enginnering plant engineer, I am well aware of your safety concerns. This switch is monitoring the air flow to the unit (commercial dryer) and when the air filter is getting dirty. The unit will still run but there is no indicator to the operator that the heat has been shut off. I am trying to install a small led to tell the operator that the filter needs cleaning (under normal conditions the filter is cleaned 2-3 times a shift). So, something simple that will not effect the normal operation of the unit is what I am looking for.

Thanks again,

Robert

[audioguru]

Hi Robert,
You don't say what is the switch's current, so I don't know if there is enough to light an LED. You also don't say what is the threshold voltage of the control logic input for the switch.

Couldn't you replace the single pole switch with a two pole switch?

[WildWest]

I actually thought about replacing the switch but it is not a standard micro switch. The actuator is a custom piece as well as the enclosure so attempting to change the switch might be more difficult than it appears. Couldn't really measure the current to the switch but I expect it is low. The switch wires are 22 ga. so not alot of current. As for the threshold voltage on the logic board, all I can tell is that it is at +25vac or GND. I still have not been able to locate a good set of schematics for the controls on this machine so I am just going by what I can measure.

Thanks,

Robert

[audioguru]

Hi Robert,
It is unfortunate that the signal is high-voltage, AC and probably low-current. Then in order to detect it and drive an LED you will need a resistive voltage-divider, rectifier and filter, opamp and power supply for it.
Wire a 1M potentiometer as a rheostat and connect it across the switch contacts when they are open. Turn the pot slowly until the system detects that the switch is closed but your pot is causing it not the switch. You might have to turn the pot very slowly or in small steps because the system might have a delay. Remove the pot, measure its resistance and let us know.

[WildWest]

Hi All,

Okay, I finally got around to measuring the trip points as requested.  As best as I can tell, the circiut tripped at approx. 50K ohms and reset at around 1k.  I still want to build an indicator that mimics the air flow switch, so any ideas will help.  I would like for it to be able to run off battery power if possible.

Thanks Again,

Robert

[audioguru]

You don't want to power anything from the voltage across the switch. Its current would fool the circuit to thinking the switch didn't open. :o

[Guest Alun]

I was about to suggest a triac but I have a feeling that the trigger current required would be too high. This circuit should work as long as the voltage across the switch is AC too. The diodes (orange) across the MOSFETS are built in so you don't need to add them, the 19V zenners protect the gates from positive and negitive voltages above 20V.

[Guest Alun]

oh and here's the schematic.

[Guest Alun]

It still depends on the impedance of the source and the load as well as the mimimum voltage requirement for the load. A triac could be used with a 4.7K resistor on the gate connecting to the switch if it needs a low impedance to trigger (or a  low current load if a very low impedance is required), but if it's high impedance you want MOSFETs are the only way to go.

What do you estimate the source impedance to be tnk2k?

I suppose you could use a bridge rectifier with a small capacitor driving high brightness flashing LED providing the flasher chip can handle the peak open circuit input voltage, if you require something brighter you could connect some non-flashing LEDs in series with it. You might also be able to buy a small low current piezo siren to connect connect in paralell.

[audioguru]

In further checking the switch is not actually in series with the gas valve, it is an input to the control logic PCB.

Hi Tnk,
You missed this part. The control logic uses very low current.