That fuse in the radio, will alert you to the simple automatic device, scheme of which was published the magazine “Radio Fernsehen Elektronik” (Germany). While a fuse F1 (see figure), the voltage between the emitter and base of transistor V1 is equal to zero and a semiconductor device is closed. Once the fuse will burn immediately on the basis of V1, a voltage is applied, opening the transistor and the led B1, included in the collector circuit will light, indicating an “accident”.
Depending on the type of led and the supply voltage select the resistor R1 so that the transistor is fully opened. If the voltage exceeds 4.5 V, in series with the led you want to include additional ballast resistor, limiting the current through Q1 to the maximum value.
Transistor V1 — any silicon, low-power: КТЗ61, КТ326, КТ208, КТ203. The diodes V2, VЗ — Д219, Д220. Led B1 — АЛ102 with any alphabetic index.
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