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need help with a wearable circuit design

inventorJohn

New Member
Hi, thanks for helping. I need to design a circuit with small basic parts and running on a small battery (3v cell?)(potentiometer)(red and green LED)(diodes?)

I have a variable resistor (potentiometer) that is set to go from 2.1Kohms up to 3.5kohms. I want a green LED to light up when the resistance is at about 3.0Kohms or maybe it can be tuned to light up at a specific resistance between 2.1Kohms-3.5kohms. A red LED would light up when outside of that range, either below or above. I want to use the least amount of parts and the smallest battery as it's for a wearable design. Can someone help me figure this out?
 

unclejed613

Well-Known Member
Most Helpful Member

rjenkinsgb

Well-Known Member
Most Helpful Member
A "window comparator" circuit should do that.
There is an example configuration about a third of the way down this page:

An LM393 dual comparator should be suitable.

Connect the variable resistor in series with another resistor, eg. 3.3K, across power, and feed the window comparator from the junction.

Connect the green led, a normal diode and a resistor in series across power - LED cathode to ground, resistor to positive.
Connect the red LED between the comparator outputs (cathode) and the junction of the diode and resistor.

You probably need more like 4.5 - 6V for it to work properly.
 

inventorJohn

New Member
Thanks!! Can someone show me a schematic layout for my specific resistance range-2.1Kohms up to 3.5kohms (and/or exact components to order? I have very little experience but think I can do this on a breadboard
 

inventorJohn

New Member
A "window comparator" circuit should do that.
There is an example configuration about a third of the way down this page:

An LM393 dual comparator should be suitable.

Connect the variable resistor in series with another resistor, eg. 3.3K, across power, and feed the window comparator from the junction.

Connect the green led, a normal diode and a resistor in series across power - LED cathode to ground, resistor to positive.
Connect the red LED between the comparator outputs (cathode) and the junction of the diode and resistor.

You probably need more like 4.5 - 6V for it to work properly.

Hi, I purchased the LM393 but am stuck on what other components are needed and how to put them together (sorry, I'm very new to this, but hoping to make this myself). I have a variable resistor (potentiometer) that is set to go from 2.1Kohms up to 3.5kohms. I want a green LED to light up when the resistance is at about 3.0Kohms or maybe it can be tuned to light up at a specific resistance between 2.1Kohms-3.5kohms. A red LED would light up when outside of that range, either below or above. Can someone help me select the components needed and show me the layout?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
This is the basic configuration for a window comparator:

jMQEG.gif

Use appropriate value resistors to set the upper an lower reference voltages to give the window between them.

If you had the pot connected between input and ground, with eg. a 2K7 resistor to V+, the wanted range would be about mid supply.

Use eg. 10K upper and lower R, with 2k2 for the centre one.
That would give thresholds at 45% and 55% of the supply.

The pot and 2k7 would give 43% at 2.1K and 56% at 3.5K, so pretty close to the same points.

Make the RL eg. 10K, fit the red LED and a series resistor in parallel with that, then add a small FET (eg. 2N7000) with the gate to Vout, source to ground and the green LED with its series resistor from drain to V+

You need at least 4.5V and preferably 6V or more for good operation.
 

AnalogKid

Well-Known Member
Most Helpful Member
From this drawing, take the center schematic.
Delete U1C.
Connect the red LED cathode to the point marked Vout in post #6.
Delete RL in post #6.

The combined circuits gets you a window comparator with one output that switches two LEDs without an additional transistor. If the polarity is backwards, you can swap the connections to the two LEDs

ak

LED-Switch-1-c.gif
 

danadak

Active Member
An alternative is to use an ATTINY85 micro -

1) Runs down to 1.8V, and can run directly off a 3V battery solution.
2) Using its A/D can measure Vbatt, then use that to determine what
value pot is at by reading its V in a simple divider network off Vbatt.
3) Can run in sleep mode to maximize battery life. << 1 uA



4) Can PWM LEDs to take advantage of eye persistence lowering needed
drain of LED when it is on, also extending battery life.
5) Pot + 1 R + Attiny85 + 2 LED + 2R + 1 Cap


Regards, Dana.
 
Last edited:

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