Double functionality on Micro

[Rajinder]

Hi all,
I have a issue where due to lack of i/o lines i need to double up on one i/o line. As an A/D to read light sensor and output to drive the LED.
The light sensor output should go from 0 to 2.7V. However, it is being clamped to around 2V (forward volt drop of LED) i think it may be due to the 220R and leakage current of the LED.

My possible solution was to use an Zener instead of the 220R or a diode instead of the 220R. I have no other i/o lines available.

Any help would be appreciated.
Thanks in advance.

 

[Rajinder]

I wil upload a schematic too.

 

[Rajinder]

Here is the schematic.
any help would be appreciated.

 

[Harald Kapp]

This will not work.
The 33 kΩ resistor will reduce the current through the LED to a mre 100 µA (assuming a 3.3 V output pin.

There are better options to increase the numbe rof available I/O lines. E.g. use a shift register to serially output a number of bits using only 2 wires (data and clock). Or use an I/O-expander, e.g. one of those offered here.
I recommend that you do not mix analog functionality and digital functionality as this may have consequences with regard to signal integrity that are hard to fix.

 

[Rajinder]

The 33K was used to stop the LED working when measuring current..
When it is a output it lights very briefly to flash every 1 minute.
Would putting a diode in place of the 220R help? Or a zener diode?
The only other thing i was thinking was to use a i/o expander as you mentioned. However i really am restricted by PCB space.
Any other suggestiins?

 

[bertus]

Hello,

What micro are you using?
What is on the other I/O lines of the micro?

Bertus

 

[Rajinder]

Hi
The other pins are used for push buttons, LEDs, analogue inputs and buzzers.

 

[Rajinder]

Its a PIC16F1938..

 

[bertus]

Hello,

How many buttons do you use?
You could multiplex buttons and use less pins in section 3-8.
This page shows some I/O situations:
https://www.mikroe.com/ebooks/pic-microcontrollers-programming-in-assembly/io-ports

Bertus

 

[Rajinder]

I have 2 buttons, the leds are already multiplexed.
I think it may come down to using a shift register, this would mean trying to free up 3 i/o lines, clk, data and latch.
Which i cant afford to do.

 

[bertus]

Hello,

How many leds are there in your project?
Ever heard about charlieplexing?
https://en.wikipedia.org/wiki/Charlieplexing

Bertus

 

[Rajinder]

I have 2 backlights for a lcd and 1 extra led. Trying to connect from 2 i/o lines. Lcd have common anodes
They all have to work independently.