Electronic hourglass. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches
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The Romanian magazine "Technium" offers an electronic timer circuit that operates on the principle of a 5-minute hourglass with a countdown every minute.
The indication is provided by LEDs. Such a device will find application in a home photo lab, in sports training and rhythmic gymnastics.
If the time interval exceeds 5 minutes, the device, like a regular hourglass, can be restarted and continue counting. Here's how it works.
(click to enlarge)
After a short closure of the SВ1 button (see the circuit diagram), the capacitor C1 starts charging, which is included in the collector circuit of the transistor VT1. The voltage from C1 is supplied to an amplifier with a large input impedance, assembled according to the scheme of a composite emitter follower on transistors VT2-VT4. Its load is an indicator on the HL1-HL5 LEDs, which turn on alternately with a minute interval.
To compensate for the initial voltage drop in the emitter-base sections VT2-VT4, capacitor C1 begins to charge from a voltage of approximately 2,7 V, which is formed on the zener diode VD1. It can be replaced by three or four series-connected silicon diodes of the D220, D223 type.
Capacitor C1 is selected with low leakage currents. The transistors shown in the diagram can be replaced with domestic ones: VT1 - KT361B, VT2 - KT312V, VT3 - KT603A, VT4 - P701A or KT801A. It requires a radiator with a dissipation area of at least 20 cm2.
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