Photo resistor in place of a vacuum sensor on a vape circuit

I'm not very educated in circuitry but I am trying to use a photo resistor (only has positive and negative) in place of the vacuum sensor(has positive, negative and ground on a disposable vape. The vape consist of a lithium battery, heat coil and a vacuum sensor. I am trying to make a light activated heat coil. I cut off the vacuum sensor and replaced it with the photo resistor and the coil won't heat up. I tried to find a place to add the ground wire but no luck. I'm wondering if the photo resistor isn't allowing enough current through to heat the coil. How can I make this work?

by Ledol
April 06, 2023

The photoresistor (or is it a phototransistor? ) is probably with a too high resistance even when on, or, with a phototransistor, quite probably not allowing enough current through it (typically, only few milliamps).

Instead of installing it in series, try to install it in parallel with the coil, as it is often seen.

I cannot check the values of the resistors, neither see if the proposed BJT (a 2N2222) is right, or not (taking too much voltage, or too much heat). Consider them as place holders rather than recommendations.

by vanderghast
April 06, 2023

Thank you!

by Ledol
April 06, 2023

1 Answer

Answer by ErnestThornton

Have you found the answer?

+1 vote
by ErnestThornton
April 20, 2023

Not quite. I'm wondering if I need to get a photo resistor that doesn't have as much resistance but I'm worried that I'm going to fry it.

by Ledol
April 20, 2023

It is unlikely that you could find a photoresistor at less than 1k when in a bright room (maybe as low as 300 ohm under the direct sunlight of an overcast day, but not much below).

Since the coil is itself of low resistance, even with a one ohm resistor and, say 9 ohm for the coil, that could make 5 volt / 10 ohm = 0.5 A. Times one ohm, gives 0.5W through the resistor (and 4.5W through the coil). Smaller is the resistor like 0.1 ohm ( as for a NMOS transistor, with Rds_on that would roughly make ten times less watt 50mW, spent at that "resistor" so smaller it is...), better it can be, even for the resistor; after all, a wire is of ... 0 ohm, almost.

by vanderghast
April 21, 2023

Change the 0.5W to 0.25W, the power is R I2.

by vanderghast
April 21, 2023

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