555 timer triggering mosfet but opto device not working

Thread Starter

dcd528

Joined Jul 28, 2011
46

I have this circuit. When the mosfet is triggered by the variable resistor on the bottom right it works great. When the mosfet is triggered by the 555 timer is works great. but when I try to combine the two the 555 dominates and the variable resistor (a light sensor) doesn't work properly. When both inputs are active the gate voltage oscillates but with a smaller variation than the 555 alone. What I would like is for the 555 to be the main controller and the light sensor to over ride the 555 when it is activated. If I bypass the light sensor and feed V+ directly to the gate with the 555 running it over rides fine. thanks in advance.
 

Alec_t

Joined Sep 17, 2013
14,206
What I would like is for the 555 to be the main controller and the light sensor to over ride the 555 when it is activated.
Define 'activated'. If you mean when the light sensor signal is above or below some threshold then you need a comparator responsive to the sensor signal.
 

Thread Starter

dcd528

Joined Jul 28, 2011
46
By active I mean that the condition turns on the load which the mosfet controls. A diode between the 555 output and the mosfet gate improves the situation but doesn't fix it. What i mean is the variation in voltage to the gate with the 555 input and the diode is a smaller variation but it still varies. Without the diode the voltage drops to near zero but with the diode it drops to about 0.5 to 1 volt.
 

Alec_t

Joined Sep 17, 2013
14,206
What light sensor are you using? One which applies a slowly varying output voltage to the FET gate could cause the FET to turn only partly on/off and generate heat which might cause the FET to fail. A comparator would overcome that problem.
 

dl324

Joined Mar 30, 2015
16,669
By active I mean that the condition turns on the load which the mosfet controls.
If you're going to talk electronics, MOSFET is an acronym, not a noun, and should be all caps.
A diode between the 555 output and the mosfet gate improves the situation but doesn't fix it. What i mean is the variation in voltage to the gate with the 555 input and the diode is a smaller variation but it still varies. Without the diode the voltage drops to near zero but with the diode it drops to about 0.5 to 1 volt.
What diode? What MOSFET? I assume it's the three terminal device, but we can't tell what polarity it is or how you have it connected.

Put some part numbers, component values, and supply values on the schematic, and assign designators for all components so we can talk intelligently about your circuit. It's better to use a logic/component symbol than the package.
 

Thread Starter

dcd528

Joined Jul 28, 2011
46
I'm obviously a casual tinkerer so my apologies if I don't follow protocol.
The MOSFET is digi key part STD150N3LLH6, MOSFET N-CH 30V 80A DPAK
In the 555 circuit R1 is 10K pot, R2 is 1K, the cap is 100 uF electrolytic.
The MOSFET has a 330 ohm resistor at the gate and a 50K resistor going to the source.
The diode was a part in my bin don't know the particulars. The variable light sensor is high resistance in the dark low resistance in light.
Without the 555 the MOSFET turns on in bright light.
What I would like is for the 555 to make the MOSFET turn on and off based on the 555 timing cycle but if there is a bright light then turn the MOSFET on as long as the light is on. Currently the 555 dominates and in bright light the MOSFET still follows the 555 and ignores the light sensor.
 

dl324

Joined Mar 30, 2015
16,669
I'm obviously a casual tinkerer so my apologies if I don't follow protocol.
Still don't understand how you have the MOSFET connected. Kindly annotate the values to your "schematic" and label the MOSFET terminals.

We use the symbol below for an N channel MOSFET:
upload_2017-6-6_11-5-58.png
And a symbol like this more readily conveys timer functionality:
upload_2017-6-6_11-7-12.png
 

dl324

Joined Mar 30, 2015
16,669
The upper leg of the MOSFET is the gate, the lower leg is the source and the drain goes to load.
This is what I got from 3 posts to describe your circuit (I took the liberty of removing the short on the gate resistor):
upload_2017-6-6_13-6-49.png
You are low side switching a load. What is the purpose of the 50k resistor on the source?

What is the load that you're switching?

How are you switching between the timer output and the LDR? What is the dark and light resistance?

Under what conditions do you want the LDR to override the timer? In that case, do you want the load always on or off?
 

Thread Starter

dcd528

Joined Jul 28, 2011
46
Thanks for you time and effort.
The 50K resistor is a pull down to shut off the MOSFET when the trigger goes low. I had latching problems without it.
The load is a DC motor, 5V.
I'm not switching between the timer and the LDR. When the light comes on I want the motor on with full power when the light is out the motor responds based on the timer output.
My first thought was the 555 was interfering with the LDR output so I put a diode on the output but that didn't solve the problem.
 

EM Fields

Joined Jun 8, 2016
583

I have this circuit. When the mosfet is triggered by the variable resistor on the bottom right it works great. When the mosfet is triggered by the 555 timer is works great. but when I try to combine the two the 555 dominates and the variable resistor (a light sensor) doesn't work properly. When both inputs are active the gate voltage oscillates but with a smaller variation than the 555 alone. What I would like is for the 555 to be the main controller and the light sensor to over ride the 555 when it is activated. If I bypass the light sensor and feed V+ directly to the gate with the 555 running it over rides fine. thanks in advance.
This should work for you:


Light detector.png
Do you need a circuit description?
 

Thread Starter

dcd528

Joined Jul 28, 2011
46
I think I get it, the comparator senses the LDR and triggers the 555 to be on all the time. Is that correct?
 

dl324

Joined Mar 30, 2015
16,669
The 50K resistor is a pull down to shut off the MOSFET when the trigger goes low. I had latching problems without it.
You shouldn't need the 330 or 50k resistors. The NE555 timer will sink or source current.
The load is a DC motor, 5V.
You need a snubber diode on the motor.
I'm not switching between the timer and the LDR. When the light comes on I want the motor on with full power when the light is out the motor responds based on the timer output.
Then you need the LDR to cause a circuit to pull the trigger input of the timer LOW; that will force the timer output HIGH. You could use a comparator, an inverter gate, or a transistor.
 
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