Romanian journal "Technium" offers a diagram of the electronic timer, acting on the principle of the 5-minute hourglass with time through every minute.
The indication to carry out the LEDs. Such a device will find application in home photo lab, sports training and exercises in rhythmic gymnastics.
If the time interval exceeds 5 minutes, the device as normal hourglass, you can restart and continue the countdown. Here is how it works.
After a short-circuit button SВ1 (see circuit diagram) begins to charge the capacitor C1 included in the collector circuit transistor VT1. The voltage on C1 is fed to the amplifier with high input resistance, collected under the scheme a compound emitter follower at transistors VТ2 - VТ4. His load is light on the LEDs HL1-HL5; activated alternately with a minute interval.
To compensate for the initial voltage drop across sites "emitter-base" VТ2-VТ4, the capacitor C1 begins to charge with the voltage about 2.7 V, which is formed on the Zener diode VD1. It can replace three or four series-connected silicon diodes type D, D.
Capacitor C1 is chosen with small leakage currents. The transistors shown in the scheme, you can replace domestic: VT1 - CTB, VТ2 - CTV, VТ3 - CTA, VТ4 - PA or CTA. It requires a radiator with an area of scattering of at least 20 cm2.