I've rebuilt the frequency generator circuit and it's working pretty much to spec, although I am using different value for R1, R2 and C this time. I did try to reproduce the behaviour I was seeing earlier, but couldn't. As others have commented, the frequency is pretty stable across the range of voltages, so I am going to put my earlier experience down to poor circuit build - I am using a breadboard so maybe one of the connections wasn't quite tight enough, or it could have been a dud cap, or.....
I'll install Pspice so I can draw up the circuit next time, but for anyone who is curious, it is the standard 555 astable circuit I am using.
The reason for building a freq gen is that I want to drive an IR diode in such a way that it's signal can be picked up by a 'tuned' IR receiver. The component I am using for the receiver is the RPM7100 series from Jaycar
https://www.electro-tech-online.com/custompdfs/2009/05/ZD-1952.pdf
and what I was hoping to be able to do is to detect the presence/absence of the IR beam using this receiver. When an IR beam with matching frequency is detected, the output of this receiver should go low, otherwise it should be high (around 5 volts).
However, I have hit a couple of challenges...
1) reading the datasheet for the receiver, its not clear to me what frequency it will detect at - the datasheet describes 4 different frequencies. The only identifying marks are on the back of the receiver - 1B34 04A.
Question: Can someone tell me categorically what frequency this IR receiver is tuned for?
2) I am using a BC549 NPN transistor with the o/p from the 555 wired to base, and an IR LED with (current limiting) series resistor connected between +5v and collector. According to my theoretical knowledge, this should cause the IR diode to switch on/off at the same frequency as the o/p from the 555.
However, I am seeing a drop in the frequency when measured at the LED, for example, if I measure the o/p of the 555 I get 36KHz, but when I connect the o/p to the transistor base, and then measure at the collector, I get readings around 28 - 30 KHz, and it is quite unstable.
Question: I'm sure there's an easy explanation for why the frequency will vary in this way. Do I need to compensate by increasing the 555 frequency, so that it will be say, 36KHz, when measured at the LED? What about the way the frequency shifts - is there something else I need in my circuit to enable a constant frequency output from the LED?