Is there another, better way of doing this? (simple circuit diagram)

wannab

I need one small input to switch on a slightly larger circuit permanently, until I cut the power off or until I open the switch. Relays seem a bit expensive and intended for higher voltage applications than I require. Is there a way to swap the relay here with transistors or something?

Simon Bridge

Depends on how you want to control the switch and under what circumstances.
i.e. you could short-circuit the main one with a wire, and put a small explosive charge (like a firecracker) under the wire. Use a NiCad element to light the fuse.

You could use a knife-switch and a servo.

Since this is to be a "once on always on" device...
You could use a mousetrap as a spring-loaded knife-switch, held open by a bit of fuse wire - blow the fuse and close the switch. (There's a lot you can do with spring-loaded switches.)

But what you do depends on the circumstances.

wannab

Sounds like I'll constantly need to replace components which isn't really what I want. I just want the switch to flick when there's a current so that the lamp and the buzzer stay on until someone deals with it.

Simon Bridge

Ah - so you need to be able to switch it off as well.
That's not what "permanent" means.

There are any number of switching circuits ... depends how it is to be triggered.
Something provides a current when the lamp needs to glow? Presumably not enough current to light the lamp by itself - hence the relay.

What's wrong with just using a transistor?
Something more robust: trip a physical switch with a servo or a solenoid.

skeptic2

What is your voltage?
How much current do you want to control?
How much is too expensive?

Could you please explain your symbols?
Is the inductor in the collector line supposed to be the coil of the relay?
If so, what does B represent in the switched circuit?
What does the symbol of the circle with the X in it represent?
Why do you have a resistor in the emitter line and what is its approximate value?

wannab

The problem with a transistor is that it only provides current while it has inputs, afaik you can't arrange them so that an impulse causes a permanent flow of current. Or am I wrong?

wannab

Very small, 6V or something. I haven't bought components yet.
Not very much. Just enough to light a LED and sound a buzzer.
Well a relay is around £5 on eBay and I thought that was too much. It was also much larger than I expected for such a simple device. If there's a way to do it with transistors that'd be great.

X is a lamp, B is a buzzer.
Is the inductor in the collector line supposed to be the coil of the relay?
Yes.
I'm not sure if it's completely accurate. I thought if I didn't then it would be a short.

wannab

There's a 12V and 40amp relay on eBay for £2. Does this only work for around these levels or would a small current and slight smaller voltage work?

dlgoff

Here's a chapter about THYRISTORS with several ways to do what you want.
e.g.

wannab

Thanks a lot.

dlgoff

You're welcome.

skeptic2

If you decide to go with the SCR circuit which I recommend, I'd also recommend that you put your LED (with the proper current limiting resistor) and the buzzer in parallel. In series, when the buzzer opens, the SCR will turn off.