help with low amperage transistor switch circuit needed

[ParishPete]

I have a bite alarm and repeater box used for fishing. The 9 volt alarm (which has packed up) gave out 4.5 volts at 50 milliamps which triggered the repeater (also 9volt powered).

The new alarm is powered by a small 12 volt battery but only puts out 2 volts and as far I can measure at only 1 milliamp.

I've tried bench testing a simple transistor switch circuit and whilst that circuit works using a 1.5v battery as a trigger instaed of the alarm and 9v battery to set off the repeater, the current from the alarm just won't operate a transistor.

Is there anything I can use to boost what comes out of the alarm so that either a 1.5v or the required 9v circuit is made.

I should add that some transistors do permanently trigger due to continuity within the alarm circuit even when it is powered off.

I have a stash of transistors, capacitors, diodes, timer chips, breadboards, etc that my late father had. I've worked through most of the transistors but none operate as I require. I also have his transistor checker.

any help welcome

Pete

[Simon (Webmaster)]

It seems surprising that the alarm can't provide enough current to switch a transistor. The transistor should only need around 0.6V to saturate it. What is the input to the repeater - is it simply a case of applying power to sound it, or does it have a trigger input? Do you know the voltage / current actually required to trigger the repeater (either from the specifications or by experimentation)?

If you can give more details of the repeater I'll try to suggest a suitable interface circuit.

[ParishPete]

Simon

Thanks for the response.

I'll do some further testing tomorrow and let you know.

regards

Peter

[ParishPete]

Simon,

It's a mystery to me. I wish I'd spent more time with my dad when he was dabbling with electroncs!

Bench test: 2N3053 Transistor, 9V battery & 1.5v battery (trigger). 9v passes between C & E extension box operates. In fact a power source of 4.5, 6 or 9v will operate it.

The alarms work on the basis that when line is pulled over a wheel, a set of blades pass through a photoelectric cell which lights LEDs and sound an alarm. The voltage and current rises, the faster the wheel spins. the extension is a simple jack plug connection.

The old alarm puts out between 1 & 8 volts, new alarm between 0.5 & 2.5 volts. Both appear to be same amperage, something around 75+ micro amps.

when using a transistor circuit with either alarm, only the voltage from the alarm is being put out through the transistor, not that from an external 9v battery .

I sense I'm missing something here. Perhaps AC rather than DC is being put out since less than 4.5 volt battery won't trigger the extension but a lesser output from the old alarm will. (appears to be approx 1 volt with single bleepfrom the alam)

The new alarm does light the LEDs on the extension but doesn't set off the speaker.

The extension and old alarm have their own 9v batteries. the new alarm has a small 12v battery.

I have about thirty types of transistor. I've been through most and they all react in the same way. perhaps they are just the wrong type.

I hope the above makes some sense.

regards

Peter

[Simon (Webmaster)]

Hi

Sorry for not getting back to this sooner. I'm still not quite sure how the repeater connects to the main alarm. Is it simply powered from the main alarm (i.e. doesn't have it's own battery and takes all it's power from the main circuit) or does it have it's own battery and simply take a trigger input from the main alarm? If you have a model number / web link / instructions for it this might shed some light on this. I assume the either way there are 2 wires from the repeater unit that need to be connected to the main alarm?

[ParishPete]

Simon,

Both units have their own battery power supply and connect via mono 2.5 jackplugs. (straightforward pos/neg connection)

I'm afraid the circuitry is nowhere to be found. there is no guide on the web as to what the output voltage actually is.

The output voltage from the alarm triggers the extension which then uses its own power to operate. when the alarm stops, the extension stops although both have a LED which stays on for about 15 seconds and that works.

The new alarm output peaks at about 2.5 Volts but only partly triggers the extension. The LEDs operate, but not the speaker.

It certainly doesn't trigger a transistor. That makes me wonder if it's AC.

On my multimeter, the output reading is same 0.5 - 2.5V on both AC & DC settings, though I can't see why it would be AC.

regards

Pete

[topfer]

The most common way to use an NPN transistor as a low-side switch is to drive a current into its base with respect to the grounded emitter, and connect the load between your V+ supply and the collector of the NPN transistor. You would typically drive a voltage into the base through a resistor, and size the resistor so that you get enough base current to support the desired collector current. You will also usually put a resistor in series with the load and collector to limit the load current.

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[ParishPete]

The most common way to use an NPN transistor as a low-side switch is to drive a current into its base with respect to the grounded emitter, and connect the load between your V+ supply and the collector of the NPN transistor. You would typically drive a voltage into the base through a resistor, and size the resistor so that you get enough base current to support the desired collector current. You will also usually put a resistor in series with the load and collector to limit the load current.

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Topfer

Thanks for response. I think what you're basically describing is a circuit such as the attached.

That circuit works when the '2v pulse' is replaced by a 1.5 volt AA battery. It doesn't work when the alarm's 0.5 to 2.5 volt pulse is used. The transistor simply passes that base voltage through to the load rather than from the 9v battery.

I can't identify why on paper the circuit should work yet doesn't.

I attach also a representative circuit of the kind of alarm i'm using though the illustration does not have an output to an extension as does mine.

regards

Pete