Help needed for very low voltage trigger for TTL logic

Bezman · 2005-02-20 11:08

Hi there,
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I’m having trouble with a project I hope someone can help with. (I have more of a repair knowledge of electonics than design and I’m struggling here.)
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I have an adjustable thermometer with low and high thresholds that power a piezio. The device runs of a 1.5v battery and the best that I can measure the output voltage when a threshold is triggered is .2v the multimeter I have isn’t responsive enough to be sure.
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I wanted this to fire a logic high as a one shot pulse.
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I thought I could wire up a LM339 comparator to trigger the logic but I’ve tried a number of methods of·referencing and I just can’t get it to go.
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I tried a combination of transistors but haven’t been able to pull up the base enough too.
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Could anyone point me in the right direction to get this .2v signal to get to 5v?
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I’m getting very close to soldering the piezio back on and use a sound activated switch!!
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Many thanks in advance!!
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Jason

Nate · 2005-02-20 15:25

The LM339 is eminetly capable of doing this.· Need more info.· Please submit a circuit schematic.

It's probable your thermometer output isn't DC (that is why you are measuring .2 V)·- depends on where in the circuit the piezo driver is.· This doesn't really matter, as it can be dealt with.· Also, what are you trying to drive with the output of the comparator? (I assume a uP).

Nate

Post Edited (Nate) : 2/20/2005 4:01:10 PM GMT

Chris Savage · 2005-02-20 21:51

Jason,

·· For starters, if you're powering a device from a 1.5V battery, you aren't going to get a 5V output.· The minimum power supply voltage I have seen for an LM339 is 2V.· At 1.5V you're not even supplying it with enough power to operate correctly.· Unless I am missing something this would seem to be your problem.

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MacGeek117 · 2005-02-20 22:12

Those chips need at least 3v to even do what you want it to do. I suggest using a 3v supply.
bugg

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Q: "And why shouldn't they, there so inconvinent!"

Beau Schwabe · 2005-02-21 04:36

Assuming you have a 5V supply to other parts of your circuitry, you could try a simple level shifter.

Although I must say I am a little suspicious of your meter reading.

Calibration of the level shifter is quite easy... Providing a LOW signal at the input, adjust the potentiometer
until the output just goes LOW. Once this is set, .2V at the input will create a logic HIGH at the output.

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Beau Schwabe - Mask Designer III

National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071

Bezman · 2005-02-22 12:45

Guys this is great, I didn't expect such a response -·Thanks!

I don't know why I had it in my head that it was DC Nate, you'll be right there. I checked on AC and it still wasn't putting out much more that my digital meter could measure. I don't have any access to better test gear unfortunatly.

My idea was to set the LM339 at 5v with the rest of the circuit, would it be better to go 12v and devide the output back to 5v?

I've attached the idea around my final requirements. Looking back I did get the comparitor to fire a pulse and I'm looking to use this to turn on a relay circuit·for a 240v device. This relay circuit is·all sweet and firing from a manualy triggered TTL circuit so I just need to sort the bit from where the Piezio was attached to a flip flop so that every time it fires the motor either goes on or off as the high and low thresholds are reached. The motor is to kick in to cool and turn of before getting too cold.

Iceman, I hadn't come accoss this combination in my research and it looks good to me. I'll give your circuit a go ASAP.

Like I said I'm really not a circuit designer and your help is very much appreciated.

Help needed for very low voltage trigger for TTL logic

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