I'm a newbie, so thanks in advance for any and all help.

I'm working with my first IC chip (ADG436) in order to create an auto switching audio project. In order for the switch to work, a particular pin must be forced low. This is a 5v project.

Primary/default input is a music player (MP3 player, cell phone, etc.).

Secondary input is a handheld walkie talkie. That input will only be activated when there is an incoming transmission. In order for the secondary input to be triggered, pins 1 and 9 of the ADG436 need to be pulled low. I'm trying to use the presence of voltage on the speaker output (3.7v) to somehow trigger the switching.

The diagram that I have attached was a crude attempt by me as a newbie to figure something out. I simply tried to use an NPN transistor as a switch.... and it works.... but only if I take the base to 5v (power rail?). The 3.7v coming out of the speaker does not seem to be enough to make it switch. I've been told that I should use an Op Amp, but I have no idea how to make that happen. Still reading, but struggling to figure it out.

Can someone walk me through this?

 

Why is the speaker output exactly 3.7V?
Is that audio (AC)?

You need a 10kΩ resistor from the collector of the resistor to +5V, and a resistor in series with the transistor base (try 10kΩ) for it to work properly as a switch.
Note that a transistor in that configuration works as a inverter, so when the input is high, the output will be low (transistor ON).

 

The basic approach is ok, but it is going to take more parts. Both the positive and negative peaks of the audio must be between 0 V and 5 V, but my guess is that your audio sources are centered about GND. If so, this part will clip off half of the audio waveform. The solution are a) run the part on +5 Vand -5 V; b) use coupling capacitors and bias resistor to elevate the audio above ground.

More later.

ak

 

Gents - thanks for your reply.

When I measure the voltage on the speaker output of the walkie, it is a constant 3.7 volts when it is receiving a transmission, regardless of if there is any noise being transmitted. When there is no transmission, it is 0 volts. I apologize, but there is a 10kΩ resister to the base of the transistor.

What I think I really want to do is remove the transistor and replace it with an op amp.
Would that be a better design?
If so, what op amp would be best to use and how would I wire that up?

Again, appreciate your help!!

 

... What I think I really want to do is remove the transistor and replace it with an op amp.

Why?

 

Would that be a better design? Again, the output of 3.7v from the walkie is not enough to trigger the transistor. By using an op amp, I thought I woudl be able to boost that and ensure that the transistor is activated. Thoughts?

 

Sorry for being pedantic, but “better design” is vague. What do you think is better?

Audio is going to trigger many times. How did you measure 3.7V? If you used. DMM on DC, you’re going to get an average reading.

In my circuit, I did use an op amp to scale up the audio voltage to almost Vcc (limitation of the op amp). Then I added an RC circuit to smooth the audio output. The TC circuit did delay output.

If you measured the audio output as above, perhaps adding the filter alone would help.

 

Why is he speaker output exactly 3.7V?
Is that audio (AC)?

You need a 10kΩ resistor from the collector of the resistor to +5V, and a resistor in series with the transistor base (try 10kΩ) for it to work properly as a switch.
Note that a transistor in that configuration works as a inverter, so when the input is high, the output will be low (transistor ON).

Hi crutschow,

I appreciate your help. I'm just now able to get back to this little side project.

To answer your question on the voltage, I have placed a multimeter on the jack and when nothing is being received, it reads 0v. However, if something...anything.... is received, the voltage goes to 3.7v. I hope that helps.

I've included an update of my schematic -- I tried to add the 10k resistor to the collector. It didn't really seem to make any difference.

There is a 460 ohm resistor on the base. I tried to swap that out for a 10k, but then I couldn't get the switching to occur.

Any help would be greatly appreciate!