Electronics Questions & Answers ← browse Q&As

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?

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.

Thank you!

Have you found the answer?

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.

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.

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