How do I add transistors?

Hi, everyone.

This is a circuit called number 7(a) of Short Circuits Volume I by Jaycar. I've built this circuit several times and it works. It's an astable multivibrator with a frequency of about 1 hertz.

Today I tried putting lamps into the circuit where the LEDs go. It didn't work, because the current passing through each lamp is in the order of 10 mAmp and the lamps are rated at 250 mAmp. Although the circuit oscillated electrically, the lamps didn't light up.

To get them to light up, I figure I need to add an extra part of the circuit in place of the LED that triggers an additional pathway, which consumes much more power. I can figure how to use a relay. Would this part do the trick?

**broken link removed**

Also, how do I add a transistor to do the same job? I can't get my head around the fact that the emitter is common to both the base and collector parts of the circuit.

Richard

Just replace your transisters with bigger ones that can handel more power like this;
https://www.circuitstoday.com/24v-flasher-circuit

Richardcavell said:

To get them to light up, I figure I need to add an extra part of the circuit in place of the LED that triggers an additional pathway, which consumes much more power. I can figure how to use a relay. Would this part do the trick?

**broken link removed**

Click to expand...

The max load current of that relay is 150mA -- you said your lamps draw 250mA (they'll draw more when they're turned on before they are glowing hot). You could use a relay, or you could change your components.

The BC548 is only rated for 100mA. You can try a BC337, which can handle 500mA. Then change the 47k resistors to something more like 4k7 (to increase the base current and therefore the max collector current), and the caps to 100uF to keep the the time constant and therefore the flash rate the same. These values should just get the lamps lighting.

Just replace your transisters with bigger ones that can handel more power like this

Click to expand...

Not quite that simple. You will also need to add a current-limiting resistor in series with each LED to keep the LED/collector current well below 240mA if you want the LEDs to survive .

I can find a relay whose coil will respond to the relevant voltage and current. I understand also that I can upgrade the existing transistors, but it's not an elegant solution. How do I add additional transistors, like a Darlington pair, to carry up to 250 mAmps through a separate part of the circuit?

a separate part of the circuit?

Click to expand...

Do you mean some circuit extra to what you have shown?

alec_t,

Yes. I mean something like this. I've drawn my idea in red. View attachment 66787

You're nearly there. The extra transistor needs a base resistor and may need to be a Darlington type or a pair of transistors, to avoid loading the multivibrator too much.
Something like this
View attachment 66789

R6 will have to be chosen to suit the LED and the supply voltage you intend to use.
Q3/Q4 could be replaced by a Darlington transistor or an N-channel MOSFET.