Sound Operated Switch
Circuit diagram
Notes:
- This sensitive sound operated switch can be used with a dynamic microphone insert as above, or be used with an electret (ECM) microphone. If an ECM is used then R1 (shown dotted) will need to be included. A suitable value would be between 2.2k and 10kohms.
- The two BC109C transitors form an audio preamp, the gain of which is controlled by the 10k preset. The output is further amplified by a BC182B transistor. To prevent instability the preamp is decoupled with a 100u capacitor and 1k resistor. The audio voltage at the collector of the BC182B is rectified by the two 1N4148 diodes and 4.7u capacitor. This dc voltage will
- directly drive the BC212B transistor and operate the relay and LED.
- It should be noted that this circuit does not "latch". The relay and LED operate momentarily in response to audio peaks.
circuit from http://www.electronics-lab.com/