Maker Factory ELECTRONICS EXPERIMENTS BOX Manual page 299

Circuit 139 and 140: The two transistors configured as Darlington pair feature a very high input resistance.
This creates a high voltage drop even at very small input current, this in return switches the transistors
through and LED1 is turned on.
Fig. 3.138: Circuit 138: output level
Mixing console
Fig. 3.139: Circuit 139 – Electrometer with
LED
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