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[DPRG] anyone can explain??

Subject: [DPRG] anyone can explain??
From: DeltaGraph at aol.com DeltaGraph at aol.com
Date: Sun Jul 17 09:05:43 CDT 2005


There are a number of people on the list that could look at this circuit and 
tell you almost instantly how it works, and maybe someone will. I am not sure 
how much that would help you other that to solve your immediate problem. 

This is not a trivial circuit.

My approach is divide and conquer.

1.  I looked up spec on LM393 to find out it can only sink current. Typical 
of comparators I think. That is when "ON"  (+ input voltage greater than - 
input voltage) the outputs are pulled near ground when "OFF"

2. Think about what is happening when ComA is ON (output tied to ground) and 
when not.
    (Sketch those cases if you need to -- that will help a bunch 

3. Ohm's law needed to find ComA V+ (and ComB V-)
    You can solve for what those voltages will be for each state of ComA 
output using    simple series and parallel combinations of resistors and Voltage 
Divider Rule.
Search internet for "Voltage Divider Rule" or "Resistor Series Parallel"
4. Look at how the comparator's output state (ON or OFF) changes the 
configuration of sources (ties to +4.5)  or sinks (ties to GND) of the  series RC 
circuit which is the key to how fast the circuit switches. I searched internet for 
"RC circuit charge discharge" and instantly had a bunch of pages and sample 
JAVA scripts to show me formulas... sample graphs ...

5. Notice that ComB does not affect the timing of the circuit, but its 
threshold is modified by the state of ComA

Your capacitor is fixed at .1uF but the series resistance tied to either +4.5 
or GND depending on comparator output state is changing.  You can find 
formulas that will tell you how long that will take.

Now with this information, I would suggest you figure out how the circuit 
works -- or you could just adjust the pot and see what happens. You could use 
either a scope or you could test with target sensor to see when the circuit is 

Another suggestion would be to study or find a discussion of how a 555 timer 
That might just help with understanding of this circuit. 

- Ron Grant

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