Best transistor switch

Other Hardware General Electronics
1

I was originally trying to use a TIP120 transistor to turn on a 12v system when it received an audio signal. However, the trigger voltage for the TIP120 is too high -- it needs to be triggered by a rather small voltage and a small current. I could use an attiny or arduino as a switch combined with the transistor, but adding an arduino (plus a power source for it) seems silly for something so simple.

Are there any transistors/relays I can use as a switch with <1v as the trigger? A one-component fix would be preferable!

Thanks!

2

You can use a voltage comparator like a LM339 etc.

3

Thanks - so if I was to use the LM339, I would connect

Audio signal to pins 6 and 7 and

Output to pins 1 and 12

12v to pins 3 and 12

Is that correct if I want a small signal at pin 7 to allow 12v at pin 1?

4

See figure 3 page 4.

You select the resistors Rref and R1 to get the required reference voltage you want.
Let’s say you set this voltage to be .4v on pin 4.

Your sensor voltage connects to Vin which travels to pin 5.

Vcc connects to pin 3, lets say this is 5v.
GND connects to pin 12.

Now the output voltage, Vo pin 2, will toggle between 0v and 5v as the Vin voltage goes above and below the reference voltage of .4v which we set with the two above resistors.

Note: Your output, on pin 2, can be connected to a transistor if you need current gain to drive a relay for example.

5

You might consider if you need some delay on turn off, else the 12V system may go off during a quiet part of the audio.

6

willsilander:
so if I was to use the LM339, I would connect
Audio signal to pins 6 and 7 and
Output to pins 1 and 12
12v to pins 3 and 12

Is that correct if I want a small signal at pin 7 to allow 12v at pin 1?

No.
First off this is the first mention of an Audio signal, these are normally biased at 0V and go positive and negitave. So unless you have a split power supply you will need to bias the audio signal to half the supply of the chip.

There are four comparators in an LM339 and you only need one.
The output is open collector, so you need a pull up resistor to get any sort of output.
The current sink capacity of the output of an LM339 is very small ( 20mA absolute maximum ) so it will not drive very much. You need an adjustable reference voltage normally supplied by a pot between ground and a few volts.

So pin 12 to ground, pin 3 to 12V.

Pin 6 to a 10 pot's wiper. One end of the pot to a 47K to ground, other end of the pot to a 47K to 12V

Pin 7 to a capacitor say 0.1uF, pin 7 also 100K resistor to 12V, pin 7 also 100K resistor to ground.

Other end of the capacitor to your audio.

Pin 1 to a 1K resistor to 12V.

Then your switched output 0 or 12V appears on pin 1, but with only 20mA sink or 12mA source.

If you want more the connect your TIP120 transistor's base to pin 1.

Adjust the pot for the switching point.