stress meter

Thread Starter

nouredine london

Joined Mar 31, 2018
4
untitled.png
Good evening
I need help to deepen my knowledge of circuits calculation
if you want to give me the equations to find R1 and VR1, R2, C1, R4, R5 R6
I do not know where to start?
ps: this is a circuit of stress meter
 

MrChips

Joined Oct 2, 2009
19,269
Let's start with R6.

R6 supplies current and voltage to power IC1, LM3915 via V+ at pin-3.
With R6 = 47kΩ, the maximum current that can be supplied to IC1 and still maintain V+ at 5V is
(9 - 5.1)/47k = 85μA

That appears to be rather low.
Making R6 = 1kΩ means R6 will supply 4mA and keep V+ at 5V.

Secondly, note that your thread title is "stress meter".
This circuit measures skin conductivity, not stress.
 

Thread Starter

nouredine london

Joined Mar 31, 2018
4
Let's start with R6.

R6 supplies current and voltage to power IC1, LM3915 via V+ at pin-3.
With R6 = 47kΩ, the maximum current that can be supplied to IC1 and still maintain V+ at 5V is
(9 - 5.1)/47k = 85μA

That appears to be rather low.
Making R6 = 1kΩ means R6 will supply 4mA and keep V+ at 5V.

Secondly, note that your thread title is "stress meter".
This circuit measures skin conductivity, not stress.

it doesnt clear for me ,,
 

Sensacell

Joined Jun 19, 2012
2,435
I have a feeling that many of these values were empirically derived.

The input itself is not defined in any way.
No equation in the world could help you unless you first quantify what the input would likely be.

See what I mean?
 

wayneh

Joined Sep 9, 2010
16,102
if you want to give me the equations to find R1 and VR1, R2, C1, R4, R5 R6
My brief 2¢.

R1 – Value chosen to limit current to transistor base in the event of a short at the touch pads (which is why you have D1 as well). Value is arbitrary beyond that, because current is also limited by R2 and the two VR. You could probably just eliminate (bypass) R1, but it's fine the way it is.

R2 – Probably determined empirically. This biases the voltage down but it's not obvious to me why it's needed. Could be part of filter with C1

R3, R4, R5 & C2 – It's not obvious to me what's going on here. I'd have to study the data sheet and how that piezo works.

R6 – Limits current to the zener diode which is acting as a voltage regulator for the input voltage to the IC. I think 47kΩ is probably too high a resistance but perhaps the designer experimented to find the highest value (lowest current) that would work.

C1 – Dampens any quick changes at the input by slowing the response of the output. (R1 may help in that regard). In other words it's part of a simple RC low-pass filter. Value is low because the current level there is low; 1 µF gives plenty of time-constant.

C3 – Stabilizes the input voltage to the IC. Value is somewhat arbitrary, plus or minus a 10X factor.
 
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