Is there a question in there somewhere?

__________________
Ron

 

Roff--

Sorry, I missed your last comment...the question was I really that I needed to figure some of it out myself. I now understand the input voltage divider and the zener diode limiter.

What is the "2.5V reference IC"? Does it make the voltage differential exactly 2.5 volts regardless of the input? What chip might this be?

At the local Radio Shack, I was only able to get a LM339 which is a Quad Comparator similar to the LM393. I made the first circuit powered first by 3 AA cells, then 4 AA cells (5.6 volts), with the same results in each case:

Findings:
1) To fire the flash- short across the flash terminals (input pin 3 to ground). This will only occur if the positive leg of the flash is attached to the circuit ground and the negative leg of the flash goes to pin 3 which is marked as the positive input. In the circuit for the LM311, the flash input is marked as the negative, while it is marked as the positive in the LM393 circuit. Does this polarity matter?

2) Relay is immediately energized upon connecting the power supply, even if flash trigger voltage goes to zero, or flash is taken out of circuit.

Components of circuit and wiring have been checked and rechecked...
I tried wiring without the transistor as in the second schemetic, but the LM339 didn't provide enough output to drive the relay at all.

The relay should not energize upon adding the 6 volt power supply when the flash is not connected, and I am not sure why it seems that the flash is connected in reverse.

I do know I don't know enough to figure out why it is not working...
Do you have additional suggestions for me to test?
I will try and secure both a LM393 and the LM311 to try things further.
Thanks

 

What pins did you use on the LM339?

__________________
Ron

 

Using the first circuit for the LM393 with the transistor driver:

This is the data sheet:
http://www.fairchildsemi.com/ds/LM/LM339A.pdf

Here are the pins for the two used:
LM339 LM393
Input(-) pin 6 pin 2
Input(+) pin 7 pin 3
Output pin 2 pin 1
V+ pin 3 pin 8
Gnd pin12 pin 4

The LM339 is a RadioShack #276-1712
I hope this info helps!
I truly appreciate the help here, the learning curve is slow...

 

The output pin for that comparator on the LM339 is pin 1, not pin 2, according to the National datasheet.

__________________
Ron

 

Sorry, but that was my typo. Of the four comparators in the LM339, I started with comparator #2, which indeed used pins 1,6, & 7. I then tried using pins 2, 4, & 5 which is for comparator #1, and got the same results in that 1) the relay is energized as soon as the power (6 volts) is applied, whether any input is attached or not, 2) the flash will only fire if flash ground is attached to input pin, and flash positive is attached to the circuit ground, and 3) when the input voltage goes to zero, the relay does not de-energize.

Short of trying a new chip, can you think of anything else why these items are happening?

 

Both circuits work the same. If you connect +6V power to the circuit with the input floating (input floating looks like the voltage is high), the relay will instantly be activated. It will be de-activated when a voltage less than about +2.7V relative to circuit ground is applied to the input, and activated if the input is greater than about +3.3V. If you need to invert the relay functionality so that
When Vin<2.7V, relay ON
When Vin>3.3V, relay OFF
let me know, and I will modify the design. I don't understand what is happening when you connect the input as you describe.

__________________
Ron

 

Ron-
It looks like I wil need to have the circuit modified and inverted for the opposite operation (thank you), but I am still trying to get the circuit to work as you have so patiently explained. I will also get some additional LM393 chips during the week as maybe the chip I am using is bad.

I rechecked the existing breadboard wiring, resistor values, and pin hookups again to make sure it is set up as in your schematic, but with no input attached at all, as soon as I connect the 6v supply, the relay energizes. If I connect (momentarily) a single AA cell as a test input (battery neg to circuit ground and battery pos to input pin), the relay stays energized. If I reverse the single AA cell polarity (battery pos to circuit ground, and battery neg to circuit input) the relay will de-energize upon momentary contact.

This is just what I was finding with the flash in that the trigger voltage before the flash occurs is 4.5 volts, and if the pos lead is connected to the input, and the negative lead is momentarily grounded, the flash doesn't fire and the relay stays energized. If I reverse the flash leads so the flash negative is connected to the input pin, and the flash positive is momentarily connected to the circuit ground, the flash will fire with each momentary contact, and the relay will also release with each momentary contact.

To me this sounds opposite to what you said should be happening, and the reverse polarity of the input doesn't seem correct either.

So, the question is, if the circuit on the breadboard is indeed wired correctly, might it be that the comparator is bad? I can take a picture of the breadboard and send it if that might be useful.

 

It sounds like R2 is not connected to the power supply, or is much too large in value (or R3 is much too low). Do you have a voltmeter? You need to measure the voltage at the junction of R2 and R3, relative to circuit ground.

__________________
Ron

 

Hi, JMB

I suggest you have a look at the following site:

http://home.cogeco.ca/~paisley4/Comparators.html

for a concise tutorial on the three comps you are working with.

Hope this helps.

__________________
All those who believe in psycho kinesis, raise my hand.

 

AllVol -- Thanks for the link, but it doesn't appear to come up correctly...the full URL didnt' come through, just the shortened version containing the "~"

Ron -- With only the 6 v supply connected, the junction of R2, R3, R4, and pin 2 (on your schematic), the voltage is 2.61 v. Connecting the single AA cell (1.34v) gives a reading of 2.85v, while connecting the flash leads (4.5v) gives a reading of 2.61v.

This might be obvious, but with the flash on and ready, putting a meter across the terminals gives the 4.5v, but naturally the terminals have to be shorted with a mimentary switch for the flash to fire. The electonics in the flash then take this voltage to zero after 90 sec. It is this voltage drop I am trying to capture and then have the relay energize at the moment, and then the reverse after this works. Thanks!

 

Forgive my ignorance about flash units. What if you want to use the flash again after more than 90 seconds has elapsed? The 4.5V is no longer there.

__________________
Ron

 

That's ok, I was trying not to confuse the issue, but the relay will trigger a pulse from the 6 v supply to another contact separate from all of this that will wake the flash back up and then the 4.5 v will be present again, and open the relay. This will all happen though an adjustable timer cicuit to be able to vary the periods of activity and inactivity.

Michael

 

OK, so when the output goes to zero after 90 seconds of inactivity, does it go to zero by turning off the supply in the flash unit?

__________________
Ron

 

Well, the display blanks, a voltmeter across the terminals will read zero volts, and if the terminals are shorted, the flash will not fire. The main power swtich of the flash has to either be turned off and then turned back on, or the "Ready" button is pressed which send the wakeup signal and the flash will be ready to fire again. When the flash is sitting on a camera, the camera will send this signal to wake up the flash just before you are taking the picture. For this off camera usage, the relay will close the circuit allowing the same trigger switch that originally fired the flash to now send the 6 volt signal to a different contact to wake up the flash. After the flash is awake again, the trigger voltage across the flash terminals would be back to 4.5 volts, the relay would open, disconnecting the pulse of the 6 volts to the extra contact point, and allowing the same momentary contact switch to now fire the flash itself. If this 6 volt voltage were to be connected to both the extra contact and the main contact, the flash would not fire (and it wouldn't go to sleep either).

Michael