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Highly sensitive relay needed

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New Member
I have a Sony projector which has a 5V 1mA trigger output. I really want to use this trigger because it outputs the 5V at the right fan cycle for a peripheral I need to run.

My problem is that radio shack (and various google relay searches) has relays for 5V but requires 89.3mA to throw the switch for the 120V circuit. I need a relay which can be thrown at less than 1mA. Would a relay that could be thrown with that little oomph close a circuit intended for 120V and about .25A?

Maybe I'm going down the wrong road with a standard relay. What else could be used?

Any help would be greatly appreciated.



Active Member
Hi wrighla,

I'm going to guess that you want this isolated,
thats why you want a relay - yes ?

5v at 1mA sounds to me like the sort of input to an
opto-isolator, which could easily operate a triac.

You may even be able to get triacs with some sort of
opto isolator built-in.

If i find anything like that, i will come back
with it.

And, yes i think that 5v 1ma would be a bit extreme
for a relay.

Regards, John :)


You can trigger a standard relay via a MOSFET. The gate drive requirements for mosfets are in the range of pA-nA. You can get mosfets designed specifically with logic-level gates.


New Member
yes, i will need the two circuits seperate. The main circuit turns on and off a 120V fan. I have to switch that somehow with the 5V 1mA.

ok, maybe a mofset is what I need. are they easy to come by (didn't see one at radio shack)? Is it relatively cheap?

thanks guys!


New Member
npn transistor. almost anny that is triggerd that low. 2n3904 is a standard one. hood the base (center pin) to the output.. tho.. should he add a resistor/diode in there, or is the output low enough? then hook the emmiter to the ground a seperate, 5 or 12 v power source, and the collector to arelays coil, which the other end up of connected to the powersource's posative. then just take the ground of your 5v 1ma and attach the ground of powersupply. then use the relay to switch 120 volts.you may also want to add a diode in reverse bias in paralel with the relay on the coild side, so that when it shuts off it wont surge the transistor... is that right?
am i right? somone wanna draw him up a circuit/ add the correct resistor/diode values?


Active Member
Hi wrighla,

I have seen reed-relays that will operate at 5v 10mA,
but i am curious as to the source of your 5v 1mA.

Why is it limited to 1 milliamp ?

If this limitation has to be adhered to,
does this Sony projector have a battery ?

If so then this 5v 1mA output would be in some way
related to this battery.

The chances are that it shares the negative connection.

You will have to check that of course,
but if so that would make things easier.

A transistor could be used to operate a relay
by using the 5v 1mA to operate the transistor,
and using the battery to power the transistor.

This would maintain the isolation from the supply.

But first check out the arrangement on the Sony.

BTW, what is a fan cycle ?

Regards, John :)


New Member
Apparently Sony uses those triggers to communicate with other Sony equipment. For example, when you put a vcr tape in a sonyt tape player, the vcr will send a signal using its trigger (via a 1/8" mini jack patch cable) to the sony tv and turn it on and set the video source. I had some JVC equipment that did this and it was called compu-link. Apparently all those components are flipping switches with that small current.

I am fairly sure there are no batteries, just because the projector is pretty old and I know they usually don't last for many years in laptops. Also, I try to stay informed on a forum following that projector (vplw400q) and I imagine that would come up. There isn't any mention of a battery in the manual. I have never opened the case on the projector and am hoping not to since dust can get in and sometimes cloud the picture.

The 1mA is what the Sony specification for the trigger. When I try to use a relay requiring ~89 mA, the voltage drops to 0. Remove the relay, and the 5V returns. So I assume the relay is sapping the small current. That's why I believe I am stuck there.

This is a home theater projector but has some fairly distracting fan noise. I have built a mfd box to enclose it, and added a seperate cooling fan to move enough air to keep the unit operating cool. The internal fan on the projector runs after the projector is turned off to cool the bulb way down. So I would need to run my external fan for that extra time as well. And it just so happens that the trigger stays 5V during the whole time the internal fan is on. That's why I want to use it.

I could hook up a battery pack if there is some way for the trigger to spark the transistor (don't know what I'm talking about here) and make use of 3 batteries to start the relay (which I know the batteries do work to close the relay). I'll do a little research on transistors.

thanks, sorry for this being so long.


Active Member
i found this.


This is a 350 Watt projector and it runs off the electric mains.

It looks pretty modern, and i am guessing it has some expensive
electronic chips inside it.
I am also guessing that one of the outputs from these expensive
electronic chips is taken out to a 'plug point' and that this
is the 5v 1mA you speak of.

It will contain a power supply for its own electronics.

I suggest that you have your separate extra cooling fan intended
for use only with this Sony unit, and fit it with a unit that the
projector can control ... with its feeble 5v 1mA signal.

This extra fan would have its normal mains lead to the electricity
supply, and also a small unit, maybe in its base, or wherever you
think suitable.

I propose a length of wire from your Sony projector carrying the
signal to your extra fan, to turn it on and off.

Such an arrangement should be fairly easy to arrange, a small supply
with a relay and transistor, suitably isolated and controlled from
the projector, built into the base of the extra fan.

Please be careful, and get the advice of someone suitably qualified.

Best of luck with it, John :)


I didn't read this whole thread, but I did see someone mention a MOSFET and someone else a PNP (or was it NPN?) Anyway, I hate BJT's cause I'm a digital kinda guy and mosfets are so much simpler to me than worrying about beta's and such, so here's what I'd suggest (and I'm still in school so there might be better ways of doing it.)

Get a normally closed relay and a P-channel mosfet from Radio Shack. Connect the "source" pin of the mosfet to your DC power source and the "drain" pin to the control pin of the relay. Connect the "gate" pin to the 5v 1mA output.

Normally, this output will be 0v so it'll turn the P-channel transistor ON which will allow the 5v signal to pass, which will OPEN the relay. When the 5v 1mA output goes high, it'll turn off the PFET and the relay will open again.

I'm still in school, so I have little hands on experience, so I don't know if its necessary, but I'd tie the "drain" of the PFET to ground with a 10k to 30k ohm resistor.

Hell, better yet...

Use the 5v 1mA output to run two MOSFET inverters from radio shack, might find them labeled as a "buffer". Much easier.

5v 1mA goes to inverter 1 input, inverter 1 output goes to inverter 2 input, inverter 2 output goes to peripherial. Don't forget to give the inverters a 5v power source and ground though ;)


Get a normally closed relay and a P-channel mosfet from Radio Shack. Connect the "source" pin of the mosfet to your DC power source and the "drain" pin to the control pin of the relay. Connect the "gate" pin to the 5v 1mA output.
N-channel is much easier to get hold of, and comes in a wider variety of power, voltage & current handling capabilities. If you use n-channel (which I would recommend), it should be connected the other way around - DC source to relay, relay to mosfet drain, mosfet source to ground, and mosfet gate to your signal. This will turn the relay on when signal is high, relay off when signal is low. If you use p-channel, it is the other way around (relay on when signal low).


i still think two inverters is best :)
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