need help with a wearable circuit design

[inventorJohn]

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]

the circuit you are looking for is a window comparator.... https://www.electronics-tutorial.ne...ircuits/op-amp-comparators/window-comparator/

 

[rjenkinsgb]

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

An LM393 dual comparator should be suitable.

https://www.ti.com/lit/ds/symlink/lm393-n.pdf?ts=1626999913716&ref_url=https%253A%252F%252Fwww.google.com%252F

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]

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]

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

An LM393 dual comparator should be suitable.

https://www.ti.com/lit/ds/symlink/lm393-n.pdf?ts=1626999913716&ref_url=https%253A%252F%252Fwww.google.com%252F

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]

This is the basic configuration for a window comparator:

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]

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

 

[danadak]

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

Technoblogy - New ATtiny Low Power

www.technoblogy.com

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.