Wiring Material

The schema operates over 6V-15V DC supply voltage. It uses positive feedback from transistor T2 to transistor T1 to keep this transistor pair in latched state (on/ off), while the state of the third transistor stage is the complement of transistor T2’s conduction state. 

Initially when switch S3 is closed, both transistors T1 and T2 are off, as no forward bias is available to these, while the base of transistor T3 is effectively grounded via resistors R8 and R6 (shunted by the load of the first gadget). As a result, transistor T3 is forward biased and gadget 2 gets the supply. This is indicated by glowing of LED2. 

When switch S1 is momentarily depressed, T1 gets the base drive and it grounds the base of transistor T2 via resistor R4. Hence transistor T2 (pnp) also conducts. The positive voltage available at the collector of transistor T2 is fed back to the base of transistor T1 via resistor R3. Hence a latch is formed and transistor T2 (as also transistor T1) continues to conduct, which activates gadget 1 and LED1 glows. 

Conduction of transistor T2 causes its collector to be pulled towards positive rail. Since the collector of T2 is connected to the base of pnp transistor T3, it causes transistor T3 to cut off, switching off the supply to gadget 2) as well as extinguishing LED2. This status is maintained until switch S2 is momentarily pressed. Depression of switch S2 effectively grounds the base of transistor T1, which cuts off and thus virtually opens the base-emitter schema of transistor T2 and thus cutting it off. This is the same condition as was obtained initially. This condition can be reversed by momentarily pressing switch S1 as explained earlier. 

EFY lab note. During testing, it was noticed that for proper operation of the schema, gadget 1 must draw a current of more than 100 mA (i.e. the resistance of gadget 1 must be less than 220 ohms) to sustain the latched ‘on’ state. But this stipulation is not applicable for gadget 2. A maximum current of 275 mA could be drawn by any gadget.

Author : Praveen Shanker - Copyright : EFY

This is a Simple Sound Operated Switch Circuit Diagram. This sound-operated switch will sense the ring of the phone and translate this to a lamp that will go on and off. The amplified signal across R2 reaches D1 through capacitor C2. The rectified audio signals provide a negative bias for Q2, a pnp transistor. This causes Q2 to conduct so the current that triggers SCR1 is provided at the gate. Potentiometer R4 sets the sensitivity. R3 and C3 delay the operating voltage for Ql so that the schema will not be triggered on by the sound of the on/off switch, SI or by the current surge. Set the lamp atop a TV receiver, turn it on and set the potentiometer so that a finger snap at two feet will trigger the lamp on. Place the speaker close to the telephone and give it a try.