I am quite new to electronics and would like a little help on something that I assume must be very common.

I want to be able to fulfill the basic function of a SPDT relay, ie. current high powers one circuit, whilst current low powers an alternative circuit.

In a relay a spring activates an alternative switch if there is no current. I can't figure out how to use no current to activate a transistor as a switch.

This must be a common requirement of circuits and as I continue to study I will probably come across the solution. In the mean time I would really appreciate someone giving me a nudge in the right direction.

Ian

 

Hello and welcome.

HERE and THERE and EVERYWHERE are sites about transistors.

Although there are many different types, in terms of your question you can think of just two - the PNP and the NPN.

With an NPN transistor, if you apply a positive signal to the base, the transistor will turn ON, and with a negative signal it will turn OFF.

With a PNP transistor, if you apply a positive signal to the base, the transistor will turn OFF, and with a negative signal it will turn ON.

You couldn't use a single transistor like a SPDT relay, but two transistors might do the trick.

Let's see how others might approach this question.

 

Google Transistor push-pull.

 

Thanks for that, it goes part way to solving my problem. I am getting a little confused by varying transistors, particularly since some suppliers use the opposite casing to others (the D the other way round).

So if I connect the bases of an NPN and a PNP together then either one will be activated depending on the polarity?

If I have an OFF-ON + pulse can I change this to a + - pulse by connecting it to ground via a suitable resistor?

 

Can you give us more details on exactly what you want to do?

 

I was thinking about using the output from an NE555 to generate a pulse. Depending on weather the output is high or low one or other set of LEDs light up.

I'm just trying to get my head around some basics at the moment. What I was really after was a simple circuit that does the basic equivalent of

If high then
circuit A
else
circuit B
end if

 

Ok, then I've done the same thing with a 555.

Assuming 5-9V power supply:

+ terminal
330 ohm resistor
LED + lead
LED
LED - lead
connection from 555
330 ohm resistor
LED + lead
LED
LED - lead
- terminal

If the pulse is "HIGH", the upper LED is shut off and the lower one conducts.
If the pulse is "LOW", the upper LED conducts and the lower one is shut off.

I made a little board with a 555, two pots for the two control resistors, 12 capacitors to allow the capacitance to vary, and the output is connected to the LED circuit above and a voltage comparator to allow connections to another circuit.

At this minute, I'm watching the LEDs alternately flash.

Top...Bottom...Top...Bottom...Top...Bottom...Top...Bottom...Top...Bottom...Top...Bottom...

 

Thanks for that. It helps to get input from somebody who knows what they are doing. I'm finding electronics to be like maths, everything you learn builds on what you've already got to grips with. But sometimes it takes a while for the penny to drop.

By the way, from your name, are you by any chance a Roman soldier?

 

I'm not sure I know what I'm doing, but eventually I get things figured out.

In terms of SPQR, I'm as WASPy as they come.
But I'm an Italophile, so I'm sure I was a Roman Senator in a prior life...