Ward pager seven

Circuit Operation Principle The ward caller circuit consists of a call control circuit, a light indication circuit, a power supply circuit, and an audio circuit, as shown in Figure 9-90.

The call control circuit is composed of a call button S2-S5, a diode VD1-VD5, a resistor RI-R7, a capacitor Cl-C6, and a relay K.
The light indicating circuit is composed of indicator lights HLl-HL8.
The power supply circuit is composed of a normally open contact K1 of the power switch S1, K, a power transformer T, a rectifier bridge stack UR and a filter capacitor C8.
The audio circuit is composed of a resistor R8, a capacitor C7, transistors V1, V2, and a speaker BL.
After the power switch S1 is turned on, the whole machine enters the standby state. When a call button in 52-55 is pressed, the indicator light is on and the capacitor is charged (for example, when the stroke is pressed, HL3 and HL4 are lit, C2 is quickly charged by VD2, S3 and K to energize K), K Power-on suction, K's normally open contact Kl and saturating, C5 is charged by VD5, R6 and K, so that K maintains the pull-in; at the same time, the AC 220V voltage is stepped down by T, UR rectified and C8 filtered, which is the audio circuit. The +5V power supply is provided, and the audio oscillator composed of Vl, V2, R8 and C7 oscillates to drive the BL to make a call sound. As the C5 charging current is gradually reduced, the voltage across C5 also gradually decreases. When the voltage across C6 drops to a certain value, K is released and the audio circuit stops working.
Component selection
Rl-R8 selects 1/4W metal film resistor or carbon film resistor for use.
Cl-C4 selects CBB capacitors with a withstand voltage of 400V; C5 selects aluminum electrolytic capacitors with a withstand voltage of 1OOV; C6 and C8 select aluminum electrolytic capacitors with a withstand voltage of 16V; C7 uses monolithic capacitors or CBB capacitors.
VD1-VD5 selects 1N4007 silicon rectifier diode for use.
UR uses 25V, O.5-lA rectifier bridge stack.
Vl selects 3AX81 type é”—PNP transistor; V2 selects S9014 type silicon NPN transistor.
K selects JRX-l3F type 6-12V DC relay.
BL uses 0.25W, 8Ω electric speakers.
T selects the power transformer with lW and secondary voltage of 5V.
S1 selects SA, 220V power switch; S2-S5 selects the moving button with contact current capacity greater than ZA.
HLl-HL8 selects 220V, l0-2OW incandescent bulb or red bulb.