Audio Source Selector Circuit

In this post I have explained a simple audio source selector circuit using a single push button and a couple of relays. The idea was requested by one of the avid readers of this blog Mr. Mike.

Design Description

The request can be seen as given below:

I need a circuit diagram to add a great feature to my spectrum analyzer project.

My spectrum analyzer will have a built in amplifier also.

What I need is a diagram for the audio source select.

I will have a RCA input and a Bluetooth connection, so I want to be able to switch between the 2 sources using only a push button switch.

Each selection will need its own LED show the active selection.

It would be nice to add a ‘spare’ selection for future upgrades.

Possibly using 5V relays to do the switching. On power up, RCA is the default input, pushing the switch once will move to Bluetooth, and a second time to the ‘spare’ and pushing a 3rd time back to RCA.

Circuit Diagram

Working Of Push Button Section

So here we see that the push button sits along with the series 10k resistor and the 1N4148 diode and the 100k resistor, and the 1uF capacitor, the whole group acts like a small cleaning team.

So when you press the push button, the 1N4148 blocks any debouncing until 0.6V is crossed, the 10k resistor try to slow down the wild bounce of the switch, and they make the clock pulse look very neat before it goes inside pin 14 of the IC 4017.

The 100k resistor quietly pulls everything to ground when nothing is pressed so nothing floats around. So when you press the button, then the IC 4017 gets one clean tick, so you get one step in the selection cycle.

Now, you can also see the 0.22uF capacitor with another 10k at pin#15 of the IC, these together ensure that whenever the circuit is powered or switched ON, the IC 4017 resets internally and its output always begins from pin#3, and is never bounced to any other random pinout.

Working Of IC 4017 Section

So now we move to the IC 4017 and we see that it acts like a tiny brain that moves step by step.

You feed the clock on pin 14 and the IC walks through its outputs.

We see that in our circuit only pin 2 and pin 4 are used for stepping and pin 7 is used for resetting, so when you press the button one time, then pin 2 becomes HIGH and this HIGH goes to the first BC547 driver.

And when you press the button second time then pin 4 becomes HIGH and this HIGH goes to the second BC547 driver, when you press the button third time then pin 7 becomes HIGH only for a moment, and this HIGH touches pin 15 which is reset, and the IC goes back to the beginning so the cycle starts fresh.

So the IC 4017 keeps walking between two steps and resetting every third press so that the audio selection does not run away.

Working Of BC547 Relay Driver Section

So here we see that each HIGH from the IC 4017 moves into a 10k resistor and then into the base of a BC547 transistor.

So when the output goes HIGH, then the 10k resistor pushes some base current inside BC547, and we see that the transistor turns ON strongly like a switch.

So now the relay coil above or below gets the +5V through the collector emitter path and the relay pulls.

The 1N4007 diode sits across the relay coil like a safety guard and when the relay coil tries to spit out a spike during switch off, then the 1N4007 eats the spike so the BC547 and the IC 4017 are safe, so this whole area works like a muscle because the IC alone cannot pull the relay coil, so the BC547 does the heavy work.

Working Of Relay Audio Switching Section

So when we come to the relay switching, we see it behaves like a gate that decides whether Bluetooth or RCA or Output or Spare should pass.

The first relay has its N/C contact connected to RCA and N/O contact connected to Bluetooth and the common goes to the final selector line.

So when the relay is OFF then the N/C connects and you get RCA selected. But when the relay is ON, then the N/O connects and you get Bluetooth selected.

The second relay works for the Output and Spare links in the same N/C and N/O fashion, so the two BC547 drivers decide which relay gets the power and the relays decide which audio goes forward.

Full Cycle Of Operation Section

So when the whole thing starts, we see that the IC 4017 is sitting in the reset state, so the first relay is OFF and the second relay is OFF, so RCA and Output are connected normally.

And when you press the push button one time, then pin 2 becomes HIGH, then the first BC547 switches ON, then the top relay pulls, and Bluetooth becomes the selected audio.

And when you press the push button one more time then pin 4 becomes HIGH, then the top relay releases, then the bottom relay pulls, and the audio moves toward Spare line.

And when you press the push button the third time then the IC 4017 output 7 touches reset pin 15, then the IC goes back to the start and both relays become OFF again, and this whole cycle then keeps repeating every time you press the push button.