Maker Factory ELECTRONICS EXPERIMENTS BOX Manual page 294

Circuit 128 to 131 The electrolytic capacitor C1 is charged via R1. Depending on the values used this process
can vary in length. Once the threshold voltage of T1 plus T2 is exceeded, the transistors turn on, LED1 is lit.
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Fig. 3.128: Circuit 128 – Switch on delay
0.5 seconds
Fig. 3.129: Circuit 129 – 2 seconds
switch-on delay