Solving Photoresistor Problem - Circuit for Motor w/ Battery Pack

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Discussion Overview

The discussion revolves around designing a circuit to operate a motor using a battery pack and a photoresistor as a switch. Participants explore various configurations and components that could be used to achieve this, including transistors and voltage dividers. The focus is on the theoretical and practical aspects of the circuit design.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant requests a circuit diagram for a motor controlled by a photoresistor, indicating they are new to the topic.
  • Another participant suggests using a transistor driven by the photoresistor, but notes that the photoresistor will limit the current through the motor.
  • Concerns are raised about the high resistance of the photoresistor even in sunlight, with a minimum resistance noted at around 100 ohms.
  • A participant proposes connecting the photoresistor between the positive supply and the base of an NPN transistor, questioning the need for a second resistor between the base and ground.
  • Another participant argues that photoresistors are not ideal for providing voltage to the NPN base and recommends using a trimpot for a voltage divider to control the brightness of an LED on the collector side.
  • Further discussion suggests that while a photoresistor could drive the base of a transistor, the lack of detailed design information makes it difficult to provide definitive guidance.
  • A participant proposes a circuit design involving a photoresistor, resistor voltage divider, and comparator to create a binary on/off operation for the motor.
  • Another participant expresses a similar need for a circuit but with a different voltage requirement for their motor, requesting a diagram based on the proposed design.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of using a photoresistor in the circuit design, with some suggesting alternative configurations and components. The discussion remains unresolved regarding the optimal design approach.

Contextual Notes

Participants highlight the importance of additional design details, such as the need for a pulldown resistor and the implications of using a voltage divider. There are uncertainties regarding the performance of the proposed configurations due to the lack of specific design data.

karthiksk
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I need a circuit diagram for running a motor using a battery pack (3 volts) that uses a photo-resistor for a switch. Really sorry for bothering you with such simple question but i am new to this thing.
Thank-you. The attached was my previous circuit it did not work.
 

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The photoresistor is just that a resistor. It will limit the current through the motor. You can drive the motor with a transistor that is switched on by the photoresistor however.
 
but the photoresistor has too much resistance even wen exposed to sunlight. The max the resistance has dropped to is abou a 100 ohm
 
but the photoresistor has too much resistance even wen exposed to sunlight. The minimum the resistance has dropped to is about a 100 ohm
 
Last edited by a moderator:
karthiksk said:
but the photoresistor has too much resistance even wen exposed to sunlight. The minimum the resistance has dropped to is about a 100 ohm

So what do you think would happen if you connected the photoresistor between the + supply and the base of an NPN transistor, where the NPN's emitter is grounded, and its collector is connected to the - side of the motor...?
 
Photoresistors are not a good source for Vin on a NPN, you need a trimpot to do a voltage divider, so you can control the brightness of the led on the Collector side
 
berkeman said:
So what do you think would happen if you connected the photoresistor between the + supply and the base of an NPN transistor, where the NPN's emitter is grounded, and its collector is connected to the - side of the motor...?

will that work properly without a second resistor between base and ground?
 
Proton Soup said:
will that work properly without a second resistor between base and ground?

Hi Proton,

This whole thread is from last summer, and wasn't very well developed. A photoresistor driving the base would be fine, probably, but without more design details it's hard to say for sure.

Your comment refers to the common design practice of providing an impedance from the base to ground, so that if there is no active drive (like from light hitting the photoresistor), the transistor is off.

Again, in this application, with so little real design data, we can't really help much on the design. In general, though, a pulldown resistor on the base would be something to think about, WRT response time, and other design considerations.
 
karthiksk said:
I need a circuit diagram for running a motor using a battery pack (3 volts) that uses a photo-resistor for a switch.

As I read the word switch I see a binary off/on operation. This circuit begs for a photo-resistor, resistor voltage divider feeding a comparator with a pot for supplying the reference voltage. The output of the comparator drives the transistor which turns on (or off) the motor.
 
  • #10
skeptic2 said:
As I read the word switch I see a binary off/on operation. This circuit begs for a photo-resistor, resistor voltage divider feeding a comparator with a pot for supplying the reference voltage. The output of the comparator drives the transistor which turns on (or off) the motor.

I have a similar need for a circuit (except mine is a 1.5v motor). Can someone post a diagram as described by skeptic2?

Thanks
 

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