555 pwm help

Thread Starter

portets

Joined Feb 10, 2012
4
so i've made a pwm motor driver circuit using 2 555's and it's working okay. 555 #1 sets the frequency, 555 #2 sets the duty cycle using a phototransistor.

my problem is that that the duty cycle is long when the phototransistor senses darkness, and i need it to be the other way around. i'd prefer not to use an inverter at the output due to added battery usage and complexity, but it's okay if i have to.


 
Last edited:

Wendy

Joined Mar 24, 2008
22,164
There are other ways to do this job. Please include the phototransistor in your schematic, that is a potential starting place.

I perfer this configuration myself, the frequency is rock stable and the PWM can cover the full range.

LEDs, 555s, Flashers, and Light Chasers

Chapter 5 - The 555 and PWM



.................................................. ........................Figure 5.3
 

bertus

Joined Apr 5, 2008
20,751
Hello,

There are three more images that give broken links.
Can you upload the images to the forum itself?

Bertus
 

Thread Starter

portets

Joined Feb 10, 2012
4
edited original post with my actual schematic.
the 3 additional images were just broken duplicates.

hey bill, in that schematic you posted, does R5 control the duty cycle? i need a phototransistor to control duty cycle.
 

Wendy

Joined Mar 24, 2008
22,164
Yes, it controls duty cycle. But don't think of it as a resistor, think of it as a voltage divider. It is a voltage input for the rest of the circuit.

Moving the voltage on pin 5 of U2a between 1/3 Vcc and 2/3 Vcc will create a smooth transition of 0% to 100%.

Depending on how you bias your photo transistor you might be able to get a linear or logarithmic value in PWM. It was one of the reasons I was interested in the photo side of the circuit.
 

Thread Starter

portets

Joined Feb 10, 2012
4
sorry, i'm somewhat new to circuitry. so does that mean higher resistance causes a longer duty cycle in that circuit? I would need it the other way around.

what do you think would be the easiest/most power efficient way to have a phototransistor control a pwm circuit so that light causes a longer duty cycle?

i'm building an analog line following robot and using it to learn about pwm.
 

Wendy

Joined Mar 24, 2008
22,164
Basically you need to set up a photodiode where it swings the most voltage for both ends of the light intensity spectrum. Some people might want to give you a math model, my suggestion is to build something then try it (measure the voltages on the output). If it doesn't work as you want it to then post it here and we'll brainstorm on it.

I'm not sure what you mean by longer duty cycle. On a fixed frequency duty cycle you can have a greater percentage on time, which is what I assume you are talking about.

Are you wanting me to go through the theory of operation on how the PWM circuit works? I have another article here that might help.

Pulse Width Modulation
 

Thread Starter

portets

Joined Feb 10, 2012
4
thanks for all of the help, and the links have answered a few other questions i had.

i found a circuit doing pretty much what i want to do:




but in the above circuit, wouldn't the duty cycle range be somewhat narrow? and when the ldr's(would be phototransistors in my circuit) are fully conducting, wouldn't it be basically shorting + to ground through the vr's?
 
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