LM3914 resistor values for 50mV peak piezo element

Discussion in 'The Projects Forum' started by BuxZED, Apr 14, 2014.

Apr 14, 2014
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I Built the above Circuit and it works like a charm.

now i want to adjust the circuit to read a range of 0 to 50mV produced from a piezo element under vibration.
also, rather than from a 9V battery, i need this circuit to operate from a 3V supply (2AA batories)

can anyone suggest how i can adjust this to work from 3V

also, do i need to amplify the 50mV signal? if so, how would i do that.

I noticed, the circuit i built draws about 7mA when it's not operating, is there a way to bring it to zero?

any help is appreciated. thanks in advance.

2. MrChips Moderator

Oct 2, 2009
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LM3914 can operate at 3V.
You can alter the range of the input by adjusting the Reference input RHI. The minimum input is 200mV. You will still need to amplify a 0-50mV signal.

Apr 14, 2014
32
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Hi
can I get away with using a single transistor or do i need to use something like a UA 741?
if it is a transistor, what transistor should I use?

4. ericgibbs AAC Fanatic!

Jan 29, 2010
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hi bux,

Amplify the 50mV signal by a factor of 25, so 50mV peak would give 1.25Vpk

Link pins #6 and #7. [ RHi = 1.25v]
Connect pin #4 to 0V [RLo=0v}
Connect a 1K resistor from pin #7 to pin #8. [~13mA per LED]

For LED Bar pin #9 to +V, for Dot pin #9 o/c

Amplified signal input to pin #5.

Use LED's with a forward voltage drop < 2.5V.

5. AnalogKid Distinguished Member

Aug 1, 2013
4,705
1,300
The 741 was a necessary evolutionary step in the analog universe, and does not get nearly enough credit around here. Still, it will not work in your application. You need a high input impedance opamp circuit based on a part that is designed for single-supply operation.

According to the datasheet, the only way to get the standby current to zero is with an on-off switch.

ak

Apr 14, 2014
32
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Hi,

what can I use to amplify the signal?

7. ericgibbs AAC Fanatic!

Jan 29, 2010
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Last edited: Apr 14, 2014
djsfantasi likes this.

Apr 14, 2014
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just for my learning, is there a reason that i can't use a transistor to amplify the signal?

9. ericgibbs AAC Fanatic!

Jan 29, 2010
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The signal will be inverted if you use a single transistor amplifier, also the output will have to AC coupled to the LM3914.
The impedance will be too low when being driven by a piezo source etc....

Its not really worth the hassle in building a transistor amplifier, when you can by an OPA for 29pence UK..

BuxZED likes this.
10. MrChips Moderator

Oct 2, 2009
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They used transistors many moons ago and results were barely satisfactory at the best of times.
Then they invented the Op-Amp.

Before some smart alec complains, yes, the op-amp was invented with vacuum tubes.

BuxZED likes this.

Apr 14, 2014
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this is where i am at the moment. any advise on how to proceed would be nice

and for my knowledge, what purpose dose R2 serve in the circuit?

12. ericgibbs AAC Fanatic!

Jan 29, 2010
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If you are following my suggestion in the previous post.? R2 is not required.

This simulation shows the MCP6001 OPA.

Have you decided which amp option.?

EDIT:
The purpose of R2 is to raise the Rhi Limit above 1.25V.

You may find this Tool I designed helpful.
http://www.electro-tech-online.com/tools/LM3914V2.php

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Apr 14, 2014
32
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Thanks for the help so far

MCP6001 looks like a cheap and cheerful solution to the problem. I'm reading the specification to figure out how to integrate this in to my circuit.

how would i intergrate a variable resister to ajust the signal voltage to peak at a value other than 50mV, for example a condition where the piezo element only produce 40mV at peak and i want that to be the reference voltage where all the 10 lights light up without having to build a new circuit.

14. ericgibbs AAC Fanatic!

Jan 29, 2010
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Look at that circuit I posted, where it says Set Gain.

Apr 14, 2014
32
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is it the RV1 in the circuit?

16. ericgibbs AAC Fanatic!

Jan 29, 2010
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I dont see a RV1 in my circuit.???

Look at this marked up image.

The 50K pot is currently set to 24K

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Apr 14, 2014
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dose MCP6001 only come in surface mount package?
is there a traditional trough hole option?

18. ericgibbs AAC Fanatic!

Jan 29, 2010
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The MCP6002 , a dual version comes in a DIP package.

BuxZED likes this.

Apr 14, 2014
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How dose this look?

20. ericgibbs AAC Fanatic!

Jan 29, 2010
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Looks OK to me.

I would link pins 5&6 on the MCP to 0V if you have no use for them.

You could use the spare half of of the MCP for future development.