I'm reading through the 'Make: Electronics' book and I just finished project 11. This project is a simple oscillator using a 2N6027 programmable unijunction transistor (PUT) making an audible (approx. 1000Hz) tone, and this tone is made to waver by using a similar but lower-frequency oscillator upstream of it. The signal is then amplified through two transistors and routed through a small 8 ohm speaker. It works great and makes a decent alarm sound. See attached schematic.

The book suggests experimenting and modifying it to your liking, so I want to add a third (very low-speed) oscillator upstream of the low-speed oscillator to modulate it and see what more complex sound(s) the circuit can make. So I made a third oscillator (not in the illustration) similar to the other two but even lower in frequency than either, and I connected its cathode (output) to the gate of the low-speed oscillator with a resistor in between. The faint gray arrow shows where. The author explained that this is how you connect the original two oscillators together, and that finding the resistor value is partly a matter of trial & error. So I connected my new oscillator to the low-speed oscillator and I tried the whole range of resistances in between - from 0 to 1 million ohms - and it didn't work.

Anyone know what I need to do to make it work? I'd love to be able to string several of these simple PUT oscillators together to make weird sounds. Thanks.

 

Yes you are doing the right thing but the "third" PUT oscillator stage might not be oscillating.

I would suggest you work on this third stage all by itself, as shown in the following image.

You should be able to see momentary voltage changes on the voltmeter. Change the values of 470K and/or 100uF until you are satisfy with the period.

After you have made this stage works, then you can connect it to the remaining stages of your previous design.

 

Thanks - I am able to get my third oscillator to oscillate by itself; the needle of my analog voltmeter moves up & down when measuring across the 10K resistor as in your illustration. Or when I substitute an LED for the resistor, it flashes on & off.

But when I connect its output (cathode) to the gate of the PUT of the low-speed oscillator in the original circuit (and having tried the whole range of resistances in between), it doesn't seem to work. I was expecting the entire siren sound to more or less raise and lower in pitch - in much the same way that the low-speed oscillator modulated the tone of the high-speed oscillator.

 

If you can get the third stage to oscillate alone, then the problem must be due to the connection to the second stage.

I think the problem is the DC level. For all three PUT to oscillate, the voltage at these three points A,B,C must satisfy the condition VA < VB < VC.

I have modified the voltage divider values in these three branches(pointed to by red arrows) so that the above condition can be satisfied.

The original design have VA < VB condition satisfied so oscillation is possible.

Hope this helps.

 

Thank you ...I got it to work based on your suggestions of changing the voltage-divider resistor values at the gate of each PUT so that the voltage is lower at the gate of each successive PUT down the line. I guess this is so that the signal is always moving 'down the line' towards the negative terminal, and not inadvertently moving back up in the wrong direction?

 

Good news.

You can now move on to another project.