Servo controlled with LDR and 555 timer circuit

SG90 servo position depends on 1ms (one millisecond) to 2ms high pulses that are spaced approx. 20ms apart. 555 timers wired in astable mode easily produce low outputs for a lot longer time than high outputs.

SG90 servo position set with light dependent resistor LDR and 555 timer circuit schematic
SG90 servo position set with light dependent resistor LDR and 555 timer circuit schematic

I didn’t measure it, but the output should be low for approx. 20ms thanks to the 100k resistor being used to set the discharge time of the 0.22µF capacitor. The servo seems to be happy with the timing.

When no light falls on the LDR, it has many millions of ohms of resistance. It is like it does not exist when it cmes to how much current flows. Approx. 1,000Ω + 10,000Ω + 3,000Ω = 14,000Ω of resistance charges the capacitor. Again, I didn’t measure it, but that appears to give the servo approx. 2ms pulses due to the position that the servo goes to.

Bright enough light on the LDR will lower the resistance to close to 0Ω (maybe 50Ω or so), so current will flow almost freely around the 10k resistor, meaning that the 10k’s resistance will be ignored when it comes to the capacitor charging. Again, I didn’t measure it, But that should means there will be about 1,000Ω + 3,000Ω = 4,000Ω of resistance setting the capacitor charging time to about 1ms based on the servo position.

Of course, a middle ground amount of light will set the LDR resistance to a middle ground amount and will fluctuate the total capacitor charging resistance between 4k and 14k.

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