R3 = 0.1 x R1
C2 = C3
C1 = 2 x C2
R4 = 3K3 (approx)
C4 = 50nF (approx)
Q1 = any NPN audio amplifier.
e.g. BC108, etc
Vcc is not critical, 6 to 12Vshould work fine. Higher or lower voltages may also work OK.
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R1 = R2 R3 = 0.1 x R1 C2 = C3 C1 = 2 x C2 R4 = 3K3 (approx) C4 = 50nF (approx) Q1 = any NPN audio amplifier. e.g. BC108, etc Vcc is not critical, 6 to 12Vshould work fine. Higher or lower voltages may also work OK. |
I am not sure of the calculations required to determine the values of R1-3 and C1-3, but below are the values that I have successfully tried, and the frequencies that they give. For the circuit to work reliably the ratios shown above should be used.
R1 |
R2 |
R3 |
C1 |
C2 |
C3 |
Frequency |
18K |
18K |
1K5 |
47n |
22n |
22n |
700 Hz |
22K |
22K |
2K2 |
22n |
10n |
10n |
1200 Hz |
The output of this circuit should be fed into a high-impedance load, say >100K, remembering that the impedance of C4 is around 3K.
A PNP transistor can also be used, with the necessary changes to the supply voltage. An op-amp can also be used, connected as follows:
| Op-Amp | Transistor |
| Output | Collector |
| Inverting Input | Base |
| Non-Inverting Input | Emitter |
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