Mosfet or OpAmp and Light ?

Thread Starter

Old but still learning

Joined Feb 9, 2024
7
Hello all,

I have a few questions after having tried to achieve with various circuits - without a micro-controller - and also various circuits with an arduino uno as micro-controller, but with my limited knowledge of electronic motor drives, and my source for the circuits designs being from various random places ( about a dozen ) found online etc.

I still have not found a solution, and have spent many hours trying.

So hopefully you more experienced persons can help!

And yes I have been identifying the type of mosfet And pin outs etc before trying them, maybe an OpAmp is more suitable? I have not tried one.

need a circuit diagram showing how to---

  1. Drive a small 12 volt DC motor (typical brushed type , like in a common battery operated drill)

2. Using the minimum amount of components and preferably no micro-controller being included in the design.

  1. Have the load/ motor receive its power as a result of the amount of light being received by an illuminated light dependent resistor. ( light source circuit is separate and not needed in this design)

4. Have this power be modulated in such a way that full illumination gives the motor full power , no illumination gives the motor no power,

And every step in between ( various light levels) could be described as a smooth ramp from 0 to 100 % depending on the amount of light on the LDR.

I have only tried this with a few different mosfets and an arduino, but just can't seem to make it happen,and also I am still confused as to proper wiring to the mosfet ( if used for this purpose and not as a high power switch only)

I really appreciate any help, Thanks!
 

sghioto

Joined Dec 31, 2017
5,390
The NE555 timer chip can be configured as a PWM generator using a LDR.
The output from the 555 can trigger a mosfet to control the motor.
 

LowQCab

Joined Nov 6, 2012
4,072
The only problem here is the definition of "Dark", and the definition of "Light".

You need to start with the Voltage of your Power-Supply.

Next find a Light-Sensor that can be implemented into a Circuit that will
vary the Circuit's Output-Voltage from around 1/3 to 2/3 of the Power-Supply-Voltage.
The rest is very simple.
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BobTPH

Joined Jun 5, 2013
8,967
I am still confused as to proper wiring to the mosfet ( if used for this purpose and not as a high power switch only)
Using a MOSFET to control a motor IS using it as a switch.

You pick an interval of time, say 1 msec. Then in each interval you turn the MOSFET on for a fraction of that. Say 500 usec, which would be half power.

This is what is known as PWM.
 

Thread Starter

Old but still learning

Joined Feb 9, 2024
7
Thanks to all 3 of you,

sghioto- the use of a 555 for pwm to the mosfet is something I had come across but the output / load power in the circuits as seen could not handle a 12 volt motor only smaller loads. Any ideas or a reference schematic?

Lowqcab -- the power supply is a 12 volt dc
I am no good at electronic equations -ex how many ohms spec. for the LDR that would be a good fit for the 12 v supply and 12 volt motor.

Bobtph- I thought ( probably wrong ) that a mosfet could be used like a rheostat a little power to the gate ,a little to the load, more to the gate more to the load
without the use of pwm ,or do all mosfets require a pwm signal to ramp up and down (I was thinking of using just a pot as a baseline , the LDR and a few other misc. components?
again probably wrong?
 

sghioto

Joined Dec 31, 2017
5,390
sghioto- the use of a 555 for pwm to the mosfet is something I had come across but the output / load power in the circuits as seen could not handle a 12 volt motor only smaller loads. Any ideas or a reference schematic?
The power handling is in the proper selection of a mosfet that can handle the load.
 

LowQCab

Joined Nov 6, 2012
4,072
A FET can be used as a crude Pot, but it will be seriously Non-Linear in response.

To design a Circuit like this REQUIRES figuring-out the Math,
and understanding Specification-Sheets.

This Project will require at least one Op-Amp, and a handful of Passive-Components.

What end-result is this project supposed to accomplish ?
Why do You want to build it ?
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Thread Starter

Old but still learning

Joined Feb 9, 2024
7
Sghioto thank you,

If you are as old as I am , you may remember the source of this circuit.

Can this circuit be used to achieve my goal assuming I chose a suitable mosfet as you have recommended, and if it would work ,exactly where and how would the LDR be placed in this circuit, and or replacing any existing components on the schematic?

as much detail as possible would be appreciated
 

Attachments

Thread Starter

Old but still learning

Joined Feb 9, 2024
7
The project will be used as a cooling fan -with the amount of air being pushed by the dc motor with fan blades attached, based on the amount of sunlight present at any time of day, so a simple common on or off phototransistor/ photocell dusk to dawn type of circuit will not suit the application.

Any help on the questions related to the uploaded schematic above would be greatly appreciated
 

LowQCab

Joined Nov 6, 2012
4,072
What are the Current-Specifications of your "12-Volt" Power-Supply ?

What are the Current-Specifications of Your "12-Volt" Motor ?
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BobTPH

Joined Jun 5, 2013
8,967
Wouldn’t running the fan based in temperature be preferred over running it based on light?

You can use a varying voltage to control fan speed, and you can use a MOSFET to vary the voltage, but PWM is preferred for many reasons.
 

Thread Starter

Old but still learning

Joined Feb 9, 2024
7

Papabravo

Joined Feb 24, 2006
21,225
Given your inability to find and or design a suitable circuit I would wager that learning and using an Arduino will get you to your goal faster, than what you are doing now. I understand your reluctance, but you need to approach this process with an open mind.
 
Last edited:

Ya’akov

Joined Jan 27, 2019
9,165
Welcome to AAC.

Could you explain why you are using a light sensor to control a fan intended to manage temperature?
Why wouldn‘t you use a temperature sensor? There can’t be a constant relationship between light level and temperature disregarding the ambient temperature and sky conditions.
 
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