Volt sensor with flashing LED

[jrcfg]

Like so many others in this forum, I am just learning electronics...bought components, power supply, breadboards, books, etc. I have built a low voltage sensing circuit using a 741 op amp which triggers a flashing LED circuit (2 transistor). The circuit works well, triggering at 12.8V (normal voltage 13.8V). I would like to add an acknowledge switch, momentary contact which will cause the LED to stop flashing and stay on as long as the low volt condition exists then extinguish when volt level returns to normal. Does anyone know if there is an IC I can use or have a circuit idea that would perform this function? My books are basic and I can't convince the wife I need some of the expensive advanced books yet. Thanks!

[russlk]

What is the low voltage point? If you send your schematic, I may be able to suggest a modification. [email protected]

[Edd]

From the voltages mentioned…sounds like you are dealing with a battery voltage monitoring situation. What I might suggest is take one step up and familiarize your self with a dedicated I.C. that is precisely suited for that manner of an application, with its very exacting crossover thresholds. The unit is the popular LM339 comparator that incorporates four individual sections within one DIP package. If you are a purveyor of freebie surplus/scrap electronics for parts there is at least a 50% possibility that you would find one utilized within the circuitry board of a chunked
switch mode power supply from a computer. Typically electrolytics and power discretes shot down the unit, not this item. Or Radimus Shackimus only wants a buck as their Catalog item #: 276-1712.<p>Your LM339 DaDa Sheet:
http://www.national.com/ds/LM/LM339.pdf<p>This unit would permit your introduction to the window comparator circuit. In such a circuit its divider references could be selected to constantly sample your DC battery voltage and give you continual monitoring of three conditions.
1. The flashing of a red LED if the sampled voltage exceeds your selected high voltage spec.
2. The illumination of a yellow LED if the voltage passes below your selected low voltage spec.
3. A constant green LED illumination if your voltage is between the specified hi and lo limits.<p>Here is your reference schematic:
http://www.discovercircuits.com/PDF-FILES/voltst1.pdf<p>The first consideration is the acquisition of a stable reference voltage from which to divide down and supply precise references to pins 5 and 6 of the 339, this is accomplished with the top LM78L09 3 term reg.
This circuit and its voltage divider fixed resistor values was designed with the monitoring of a 5VDC supply in mind , so a few changes will be in order to use it for a 12VDC level. Initially at the Vin arrow one needs to insert an ≤100kΩ series resistor between that point and your battery. This way your resultant battery voltage reference will be at ½ of its actual value and within the realm of comparing against the lower ≤9VDC referenced settings which is not comparable to the 12VDC battery voltage directly. To more precisely attain a halving of the voltage, one could also place a 10kΩ pot outer leg in series with the 100kΩ res and take off the wiper terminal Then take alt readings AT the 12v level and then PAST the resistive pair until your voltage exactly halves. Since 110kΩ should be too high, shunt the input 100kΩ res with an ≤1 megΩ fixed res so that the trim pot comes within its adjustment range to present an exact halved voltage.
With that accomplished the only thing left to do is change the fixed voltage divider reference values presented to pin5 and 6 of the comparator. As they are, the unit would be responding to an over voltage threshold of 10.4VDC on your battery and 9.6VDC on the low voltage threshold…..not exactly the values that you are needing. So, figure on not using the voltage divider resistor pair specified (11kΩ+15kΩ and 13kΩ+15k&#8486 .Instead, make up a duo of these. Use a series trio of a 4.7kΩ fixed resistor on top connected down to a 10kΩ pots top terminal and its bottom terminal connected to a 15KΩ fixed resistor which grounds. The pots center wiper terminal feeds the divided reference voltage into the 339’s pin 5(6). With the adjustable trio arrangement used instead of the fixed resistor divider scheme you should be able to set your thresholds to those that you require. If you care to fine tune them later, you can set up a voltage threshold under power conditions and then power down and take a reading of the two resistances required to attain that threshold and further fine tune it with a lower pot value than the 10kΩ .I like to use a variable pot in the order of 1/10 of the value of the whole series trio and then compute/trim in the top and bottom fixed R values .This leaves a vernier value adjustment range of 10% attainable with the pot within its 270 rotational axis….the use of 10 turn trim pot stretches it out to adjustment within a 3600 scope, but, alas, beyond your ref stability capabilities. You mentioned the use of a flashing circuit for your red LED that you are using, if it doesn’t interface properly, figure on the utilization of a dedicated flashing red LED with its internal flash circuitry on die.<p>73's de Edd
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[jrcfg]

Thanks for the suggestion...I'll have to try the 339 op amp. RussK sent me a latching circuit which I added to my breadboard today and tested with my circuit. Works great. My circuit also uses an 8v regulator for a reference voltage which is adjusted with a pot. Will have to tear it all apart and try the 339 approach. Thanks for the response.

[Edd]

RGR…RGR….I just figured that with just a wee bit more component wise , you could have a lot more info available as multiple readouts. Typically if you are floating the 8 V reg center term with a pot you are sacrificing a bit in its overall stability….(within the realm of the typical stability of 3 terms in general !) It would be Ok to use your existent 8V reg in place of the 9 V specified unit, since you have it already, and it will also have adequate level for the reference derivations.<p>73's de Edd
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