These days, most of the BEP reader interested in Audio Engineering asked for a circuit which can recognize the polarity (positive and negative terminals) of speaker. I had also experience the problem of polarity of speaker about few year ago while testing one of my project. At that time, I had used a multimeter to test the polarity of speaker.
While testing multimeter you have to concentrate on the movement of speaker cone which is difficult job specially for small speaker. For large speaker, there is also problem of limiting current which may be very small for cone movement. There is various other problem like supplying DC voltage, tedious, time consuming for mass production etc.
In order to overcome these above problem, I came with a simple circuit of speaker polarity test circuit with a flashing red and green LED for unambiguous signal to testing person (Figure 1). Not let’s talk about circuit description and it’s working procedure.
Description and Working of Speaker Test Circuit
Speaker is a transducer which convert electrical signal to sound signal. But according to Faraday’s law when we tap gently on loudspeaker cone a signal is generated at its terminal which is sinusoidal in nature. The position (positive and negative) of first half cycle of signal generated decides the polarity of speaker. Thus, the circuit given below is used to recognize the position (positive and negative) of speaker.
There are two cases to identify the polarity of speaker
- First half cycle is positive: – The signal from speaker is amplified by op-amp (IC1:C). The amplified output is given to one input of AND gate (IC2:C) whose another input is connected to pin 2, complement output of IC3: A. When amplified output is high, the output of AND gate (IC2:C) also become high which SET flip-flop (IC3:B), results LED1 flash once.
- First half cycle is negative: – The entire procedure is same as above for positive half cycle. The signal is amplified by amplifier IC (IC1:D) followed by AND gate (IC2:D) and flip-flop IC (IC3:A), results LED2 flash once.
R11 – C1 and R12 – R2 is time constant components which reset flip-flip 1 (IC3:B) and flip-flip 2 (IC3:A) respectively. These combinations are responsible for time duration of LEDs.
BEP NOTE: Labeling of LED (LED1 and LED2) for positive and negative pin of loudspeaker depends on how user defines position of LEDs.
PARTS LIST OF SPEAKER POLARITY TEST CIRCUIT
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1, R2, R5 = 68 KΩ
R3, R4, R6, R15, R16 = 1 KΩ
R7 – R12 = 15 KΩ
R13, R14 = 2.2 KΩ
|C1, C2 = 100 µF/16V|
|IC1 = LM324
IC2 = CD4081
IC3 = CD4013
D1, D2 = 1N4148
|LED1 = 5mm Red LED
LED2 = 5mm Green LED
LS1 = Speaker to be tested