Voltage drop detection - Electronic Circuits Projects Diagrams Free

jmb4370 · 7th June 2008, 04:11 AM

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 · 7th June 2008, 04:20 AM

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

jmb4370 · 7th June 2008, 04:34 AM

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 · 7th June 2008, 01:08 PM

Quote:

Originally Posted by Boncuk

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 · 7th June 2008, 01:43 PM

Hi Jmb4370,

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

http://www.intersil.com/data/fn/fn3182.pdf

on1aag.

jmb4370 · 7th June 2008, 02:16 PM

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:

http://dl.sii-ic.com.edgesuite.net/s.../S808XXC_E.pdf

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 · 7th June 2008, 03:25 PM

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 · 7th June 2008, 03:32 PM

Quote:

Originally Posted by Roff

Why not use a comparator wired as a comparator?

you've got a point

jmb4370 · 7th June 2008, 03:42 PM

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 · 7th June 2008, 03:50 PM

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

Roff · 7th June 2008, 03:56 PM

Quote:

Originally Posted by Hero999

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 · 7th June 2008, 04:19 PM

Here are a couple of options.

jmb4370 · 7th June 2008, 05:21 PM

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 · 7th June 2008, 07:33 PM

Quote:

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

Quote:

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.

Quote:

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.

Quote:

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.

Quote:

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 · 7th June 2008, 08:56 PM

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.

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7th June 2008, 04:11 AM	(permalink)
jmb4370	Voltage drop detection 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

7th June 2008, 04:20 AM	(permalink)
Boncuk	Voltage drop detection Use an OpAmp wired as comparator and high resistance voltage dividers to gain a reference voltage and an input voltage. Last edited by Boncuk; 7th June 2008 at 04:23 AM.

7th June 2008, 04:34 AM	(permalink)
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

7th June 2008, 01:43 PM	(permalink)
on1aag	Hi Jmb4370, Two comparators and a reference in one "box" should do the trick. Examples are decribed in the datasheet. http://www.intersil.com/data/fn/fn3182.pdf on1aag.

7th June 2008, 02:16 PM	(permalink)
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: http://dl.sii-ic.com.edgesuite.net/s.../S808XXC_E.pdf 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!

7th June 2008, 03:25 PM	(permalink)
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? __________________ Ron

7th June 2008, 03:42 PM	(permalink)
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?

7th June 2008, 03:50 PM	(permalink)
Hero999	If you use a small enough relay then you could use an LM311 as it can power it without an external driver transistor. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez

7th June 2008, 05:21 PM	(permalink)
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!

7th June 2008, 08:56 PM	(permalink)
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.