Transmitter

The circuit is not very conventional. He spikes the tank with a LED pulser chip. Why don't you direct your questions to Tony? Post on his forum regarding this circuit. Here:

Tom

Normally, single transistor VHF transmitters have an emitter resistor. Here's an example

The transistor operates in common base mode, with the 4.7pF capacitor providing feedback from collector (output) to emitter (input).

Although that tracker circuit shows an earth symbol connected to the emitter, the "common" point is really the junction of C2, C3 and L1. Think of everything between this point and the emitter as a battery and resistor in series and you have the standard circuit.

My question was more with the use of C4. If I remove c4, the circuit should still operate. But I read on a web page that this capacitor is needed in real life because there is loss. this capacitor help keeping the voltage to the tank circuit. I just like to know how it keeps the voltage of L1 and C3. The way I see it, C3 and L1 oscillate at about 90MHz all the time, then the beep (output of the IC1) turns on the base slowing the oscillation . Therefore, c4 theoretically is not needed , right?

ken

Wrong. It is an integral part of the oscillator. If you remove it, the best you will get is some ringing (a damped sine wave) when your timer gives it its "ON" pulse.

Try googling for "oscillator circuit" or so.

Good Luck! Rich

The transistor is an amplifier. Some energy is fed from the tuned circuit, via C4, to the emitter, which is the amplifier's input. The amplifier supplies just enough energy back into the tuned circuit to replenish what is taken/lost, thereby sustsaining the oscillation. If you remove C4, you break the feedback loop, and oscillation ceases.

The output of IC1 powers the oscillator. When there is no power, there is no oscillation.

OK, maybe I take that last sentence back, having looked at the LM3909 datasheet. If there was sufficient voltage between pins 6 and 4 to keep the oscillator going all the time - and I don't know if there is - then it would oscillate "all the time" and be both amplitude and frequency modulated by the flasher. But, either way, C4 is required.