In my class we were tasked to construct the project called 'Remote Control Doorbell' from Alexan Projects and Circuits Volume 2, which are actually a bunch of projects compiled from electronics enthusiasts magazine. Below is a bad copy of some of the pages (The copy our teacher gave us to work with was already a bad copy, I could not find an original copy).
For now, let's focus only on the transmitter first:
Currently among 10+ groups in my batch that are working on this, all of us have the same problem with the transmitter: We can observe the modulating signal (somewhere between 26-40KHz) being output by the multivibrator using a frequency counter and oscillator, but there is no frequency(300MHz) being generated by the LC oscillator, even if we isolate the LC oscillator from the multivibrator by connecting the emitter of the feedback transistor Q2 straight to ground. All I can see when I try to read the output of the oscillator is a fixed straight line at 9V, no matter what the magnification and time/div I set the oscilloscope at, not even a faint trace of any kind of oscillation going on.. My group has tried reading if the the feedback amplifier transistor of the LC oscillator is properly biased so we first bypassed Q1 by connecting the emitter of Q2 to ground and this is what we read:
Base Voltage of Q2 is 1.25V
Collector Voltage of Q2 is 9V
Emitter Voltage of Q2 is at ground potential
So I'm not sure but maybe this indicates that the feedback transistor Q2 is properly biased and in the correct operating mode, which leaves me with the only culprit for the problem as to be the rest of the oscillator circuit components. I tried to check if the terminals of the component parts have continuity with the copper tracks on the bottom and they all seem to be properly connected (I placed one probe on the terminal of the component from the top side of the board, and one probe on the copper track at the bottom of the board to check this). I do not know anymore what option to explore please help us.
Also, I bought a wireless doorbell cheap from CD-R-King so I can open it and see how a properly functioning wireless doorbell should operate and maybe I could compare it with what we are making.
For now, let's focus only on the transmitter first:
Currently among 10+ groups in my batch that are working on this, all of us have the same problem with the transmitter: We can observe the modulating signal (somewhere between 26-40KHz) being output by the multivibrator using a frequency counter and oscillator, but there is no frequency(300MHz) being generated by the LC oscillator, even if we isolate the LC oscillator from the multivibrator by connecting the emitter of the feedback transistor Q2 straight to ground. All I can see when I try to read the output of the oscillator is a fixed straight line at 9V, no matter what the magnification and time/div I set the oscilloscope at, not even a faint trace of any kind of oscillation going on.. My group has tried reading if the the feedback amplifier transistor of the LC oscillator is properly biased so we first bypassed Q1 by connecting the emitter of Q2 to ground and this is what we read:
Base Voltage of Q2 is 1.25V
Collector Voltage of Q2 is 9V
Emitter Voltage of Q2 is at ground potential
So I'm not sure but maybe this indicates that the feedback transistor Q2 is properly biased and in the correct operating mode, which leaves me with the only culprit for the problem as to be the rest of the oscillator circuit components. I tried to check if the terminals of the component parts have continuity with the copper tracks on the bottom and they all seem to be properly connected (I placed one probe on the terminal of the component from the top side of the board, and one probe on the copper track at the bottom of the board to check this). I do not know anymore what option to explore please help us.
Also, I bought a wireless doorbell cheap from CD-R-King so I can open it and see how a properly functioning wireless doorbell should operate and maybe I could compare it with what we are making.
