LM3914 resistor values for 50mV peak piezo element

Thread Starter

BuxZED

Joined Apr 14, 2014
32
LM3914_5V_voltmeter_circuit.jpg


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. :)
 

MrChips

Joined Oct 2, 2009
21,687
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.

Read the datasheet.
 

Thread Starter

BuxZED

Joined Apr 14, 2014
32
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.

Read the datasheet.
Hi
Thanks for the reply.
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?
 

ericgibbs

Joined Jan 29, 2010
10,520
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.
 

AnalogKid

Joined Aug 1, 2013
8,540
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
 

Thread Starter

BuxZED

Joined Apr 14, 2014
32
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.

Hi,

Thanks for the reply
what can I use to amplify the signal?
 

ericgibbs

Joined Jan 29, 2010
10,520
Last edited:

Thread Starter

BuxZED

Joined Apr 14, 2014
32

ericgibbs

Joined Jan 29, 2010
10,520
thanks for the reply,
just for my learning, is there a reason that i can't use a transistor to amplify the signal?

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..
 

MrChips

Joined Oct 2, 2009
21,687
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.
 

Thread Starter

BuxZED

Joined Apr 14, 2014
32
this is where i am at the moment. any advise on how to proceed would be nice


Capture.JPG

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

ericgibbs

Joined Jan 29, 2010
10,520
this is where i am at the moment. any advise on how to proceed would be nice


View attachment 67213

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

BuxZED

Joined Apr 14, 2014
32
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

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.
 
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