This interesting circuit from a publication from the 1970s, measures the intensity of the sound captured by a small speaker used as a microphone. The input transformer is an inverted transistor radio output transformer and the other transformer that powers the indicator is a driver. The indicator is a 50 to 200 uA full-scale microammeter and the diodes are germanium. The circuit must operate with 6 to 9 V power and modern PNP transistors such as BC558 with small changes in the resistor values.
| Clique na imagem para ampliar |
