Voltage drop detection

[jmb4370]

The electronic flash unit is fired using a momentary SPST switch. Before the switch is pressed, putting a digital voltmeter across the switch terminals reads a trigger voltage of 4.5 volts. After a set period of inactivity, the electronics in the device cause this voltage to drop to zero.

I am trying to detect this voltage drop and then control a separate relay without affecting the flash circuit itself.

Any help would be most appreciated!
Thanks

 

[Boncuk]

Use an OpAmp wired as comparator and high resistance voltage dividers to gain a reference voltage and an input voltage.

 

[jmb4370]

Boncuk-- Thanks for the reply, can you provide more details in a schematic so I can ask some more intelligent questions? I have never used OpAmps or voltage dividers.
Thanks

 

[Roff]

Use an OpAmp wired as comparator and high resistance voltage dividers to gain a reference voltage and an input voltage.

Why not use a comparator wired as a comparator?

 

[on1aag]

Hi Jmb4370,

Two comparators and a reference in one "box" should do the trick.
Examples are decribed in the datasheet.

https://www.electro-tech-online.com/custompdfs/2008/06/fn3182.pdf

on1aag.

 

[jmb4370]

Thanks all for the start!

I have reviewed the Intersil data sheet, but couldn't find the specific IC at mouser.com. I did find many others searching under "voltage detection". The first one that came up is this one by Seiko, which looks like it should work also:

**broken link removed**

Once I get one of these IC's and start testing this setup, I will either ask more, or report the good, bad, or ugly!

Thanks again!

 

[Roff]

The simplest, cheapest, most available, easiest to use comparator is probably LM393. We can crank out a quick schematic if you answer these questions:
Do you have a relay and a power supply voltage selected?

 

[Boncuk]

Why not use a comparator wired as a comparator?

you've got a point

 

[jmb4370]

The extra power supply and relay can be anything small, likely just a 5 volt relay. I have also considered just using possibly 2 electronic relays to get the SPDT switching I need.

Does this make sense?

 

[Hero999]

If you use a small enough relay then you could use an LM311 as it can power it without an external driver transistor.

 

[Roff]

If you use a small enough relay then you could use an LM311 as it can power it without an external driver transistor.

Good point.

 

[Roff]

Here are a couple of options.

 

[jmb4370]

Roff :

Thanks a bunch! I see the second one uses the LM311 to drive a small relay directly (thanks Hero999) without the need for the driver transistor as for the LM393. I think I will get some of each to try it both ways. I can think of plenty of uses for this item!

In reading through the data sheets, and analyzing the schematics, I am trying to understand why the LM311 can drive the relay directly, while the single voltage comparator (U1a) needs the R1 and Q1 to drive the relay. Is this just to show how to do it if the relay had a heavier coil (L1), or am I missing something in the different output level provided by the LM311 vs the LM393 ?

The input pin #3 on the schematic for the LM393 is marked as + (pos), while the input pin #3 for the LM311 is marked as - (neg). Does this mean that for the LM393 circuit the input is the positive side of the flash trigger circuit, while for the LM311 circuit the input gets connected to the negative side of the flash trigger circuit?

If the supply voltage were 9 or 12 volts, would the input resistors need different values? It doesn't appear that way, as the data sheets allow a supply to 15 volts.

I appreciate your help with this so far, as being a take apart mechanical engineer, even getting to this point would have taken me the rest of the weekend!

In thinking about this for some other applications, if the differential input voltage being tested is higher than 30 volts, then is it a voltage divider on the input leg (pin 3) that would be needed to allow this to work without burning things up?

Lastly, how did you draw the schematic so nicely?

Thanks!

 

[Roff]

Roff :

Thanks a bunch! I see the second one uses the LM311 to drive a small relay directly (thanks Hero999) without the need for the driver transistor as for the LM393. I think I will get some of each to try it both ways. I can think of plenty of uses for this item!

In reading through the data sheets, and analyzing the schematics, I am trying to understand why the LM311 can drive the relay directly, while the single voltage comparator (U1a) needs the R1 and Q1 to drive the relay. Is this just to show how to do it if the relay had a heavier coil (L1), or am I missing something in the different output level provided by the LM311 vs the LM393 ?

The LM311 can sink up to about 50mA, while the LM393 can only sink about 10mA max. See the curves below.

The input pin #3 on the schematic for the LM393 is marked as + (pos), while the input pin #3 for the LM311 is marked as - (neg). Does this mean that for the LM393 circuit the input is the positive side of the flash trigger circuit, while for the LM311 circuit the input gets connected to the negative side of the flash trigger circuit?

See the annotated schematic below. If you use a form C relay (SPDT), you can use the appropriate set of terminals to energize your slave. If you use a SPST relay, and you need the relay energized when the input is zero volts, we will have to make a few changes to the circuits.

If the supply voltage were 9 or 12 volts, would the input resistors need different values? It doesn't appear that way, as the data sheets allow a supply to 15 volts.

The voltage divider on the input would need to be changed so that the switching point stays at around 2.5V. Alternately, you could change the lower 10k resistor to a 2.5V reference IC. Then it would be independent of the supply voltage.

I appreciate your help with this so far, as being a take apart mechanical engineer, even getting to this point would have taken me the rest of the weekend!

In thinking about this for some other applications, if the differential input voltage being tested is higher than 30 volts, then is it a voltage divider on the input leg (pin 3) that would be needed to allow this to work without burning things up?

Yes, or some sort of limiter, such a zener.

Lastly, how did you draw the schematic so nicely?

Thanks!

I used Linear Technology's SwitcherCAD III (aka LTspice), which is also a simulator, although I didn't simulate these circuits.

 

[jmb4370]

Roff:

"If you use a SPST relay, and you need the relay energized when the input is zero volts, we will have to make a few changes to the circuits." This is probably what will happen if I use 2 SPST relays, one would open, while the other would close. Using a single SPDT relay, then it would be energized only when the voltage is present.

I was thinking of using a 9 volt batttery to power the circuit, and provide the power for the small relay.

 

[Roff]

Roff:

"If you use a SPST relay, and you need the relay energized when the input is zero volts, we will have to make a few changes to the circuits." This is probably what will happen if I use 2 SPST relays, one would open, while the other would close. Using a single SPDT relay, then it would be energized only when the voltage is present.

I was thinking of using a 9 volt batttery to power the circuit, and provide the power for the small relay.

Is there a question in there somewhere?

 

[jmb4370]

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

 

[Roff]

What pins did you use on the LM339?

 

[jmb4370]

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

This is the data sheet:
https://www.electro-tech-online.com/custompdfs/2008/06/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...

 

[Roff]

The output pin for that comparator on the LM339 is pin 1, not pin 2, according to the **broken link removed**.