Levsha 1995 №10

Measuring the radiation can be made not only with expensive factory-made instrument. The simplest device (see Figure 1), can be build if you have a counter (radiation detector) type SBM-20 or similar. The circuit is powered by a 220 volts AC 50/60 Hz. The double-voltage rectifier circuit is made of diodes VD1, VD2, and capacitors C1, C2. The DC voltage across the capacitors C1 and C2 is about 310 V on each one, so the total voltage of the rectifier is approximately 620 V.

Figure 1. Geiger counter circuit diagram

BD1 - SBM-20 (400 V Geiger tube ); HL1 - TN-0.2 (85 V neon lamp);
VD1, VD2 - KD105B (1N4004...1N4007); VT1 - KT618A (BF469, BF471, BF393)
C1 - 0.1uF x 400V, C2 - 0.5uF x 400V;
R1 - 51k; R2 - 5.1M; R3,R4 - 100k; R5,R6 - 51k; All resistors are 0.5 Watt

The voltage 620 V is divided by a voltage divider to the 400 V. The resistors R5 and R3 forms the voltage divider for the voltage across the capacitor C1, and the resistors R6 and R4 - for the voltage across the capacitor C2. That is why the voltage across the resistors R3 and R4 is about 200 V. Thus, the Geiger-Muller counter BD1 (radiation detector) is powered by 400 V, while the neon lamp HL1 (an indicator) is powered only by 200 V. The neon lamp HL1 is lit only when transistor VT1 is on, and this transistor is on when the radiation ionizes the gas inside the Geiger-Mueller tube BD1. The resistor R1 helps to switch off the transistor VT1 when there is no radiation, and the resistor R2 limits the base current of the transistor VT1 when there is the radiation.

When there is no radiation sources around, the Geiger-Muller produces 20...30 electrical pulses in one minute because of the natural background radiation. Consequently, the transistor VT1 will be open for a short time - the neon lamp HL1 flashes every 2...3 seconds, but without strict periodicity. When the radiation levels getting higher, the neon lamp HL1 flashes more frequently. This can be verified by bringing to the counter the usual Christmas toy, covered in phosphorus. When the radiation level is very high, the neon lamp HL1 is lit continuously.

To add an audio indicator to the circuit, connect a piezoelectric buzzer, connected in series with 10k resistor, across neon lamp HL1.

Figure 2. Modified Geiger counter circuit diagram powered by a 110 volts AC

The circuit can be modified for 110 volts AC 50/60 Hz, as shown in figure 2. In the basic circuit has been added diodes D3, D4, capacitors C3, C5 (0.5μF x 400V), and the values of resistors R3, R4 has been changed to 150k.

V.BANNIKOV