Radio 1979, 04
Design of a radio receiver can be simplified by using a microammeter instead of a mechanical tuning scale. The microammeter connected across the arms of the DC bridge. The rod of the potentiometer of this bridge is connected to the rod of a variable capacitor or a tuning potentiometer of a radio receiver. Frequency scale can be printed on a paper in an ink-jet printer. As an example, the figure 1 shows a diagram of the device for a radio receiver with a supply voltage of 9 V.
V1 - 1N4732 (A Zener diode of 4.7 V); PA1 - a microammeter of 50 μA
In this circuit the arms of the bridge are formed by resistors R1, R2 and a potentiometer R3. The microammeter PA1 with with full scale deflection of 50 μA (the resistance of the coil is 910 Ohms) is connected between the midpoints of the bridge. The rod of the potentiometer R3 is connected to the rod of a variable capacitor. To reduce the nonlinearity of the scale, use a linear taper potentiometer as R3 and limit the working angle of rotation of the rod to 180°...200°.
Adjust the resistor R1 to set up the zero point of the scale, and adjust the resistor R4 to set up the upper point of the scale. The voltage regulator (based on the Zener diode V1) is used to keep the tuning scale stable (if a receiver has it's own voltage regulator then it can be used to power the circuit).
The microammeter PA1 of this device also can be used as a tuning indicator for a receiver. In this case connect the microammeter PA1 to the wipers of R1, R3 by using a multipole switch. The second position of the switch is used to connect the microammeter PA1 to the output of the field strength of the receiver