This circuit was obtained in the publication 101 Electronics Projects of 1979. The circuit can be assembled easily, since all the components are common. What this circuit does is to turn on a red or green remote LED, indicating a situation or making a call. We can use red for busy and green for free etc. The supply can be done with voltages between 3 and 9 V. R1 depends on the voltage of B1. 330-ohm to 470-ohm for 6 V, 820 ohms to 1k for 9 to 12 V.
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