Battery level indicators generally detect the voltage levels of the batteries to give a result. So, tester circuits must not be a heavy load during the measurement process.
This tester circuit draws very low current. A short duration of LED bright will show you the battery has still enough voltage level to operate devices. This light brights due to the discharging of C1 on D1 LED, this happens only when the battery provides enough voltage.
When you close the S1 switch, Tr1 transistor makes C1 to discharge through R3 current limiter. Minimum required battery voltage level can be determined by using voltage divider R1/R2. Values of R2 and R3 must be calculated as shown below;
R2 = (0.6 x R1) / (Vbmin – 0.6) Ohm and R3 = (Vb – 1.4) / 0.2 ohm
For example, for 6.5Vb(min) value (to test a 9V battery) R2 must be 10k and R3 must be 39 ohm. R4 must be between 10k and 1M.
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