EasyEDA first order problem.

[Nigel Goodwin]

Filtering the PWM for the contrast didn't work with a 4k7 resistor and a 10uF capacitor. The contrast pin appears to supply a current as even switching the output to zero resulted in 1.5V on the contrast pin. To fix it, the two components in the top right are the resistor and capacitor, so I soldered a 1K 1206 resistor diagonally between the bottom of the resistor and the top of the capacitor and all works well.

As I mentioned in post #12, I used a 1K and 10uF for PWM, which has always worked fine - however, nothing 'clever' about it, I just copied the values from an example I'd seem somewhere on-line.

Interesting to hear that using a higher value resistor messes things up.

Nice looking board though.

 

[Pommie]

I'd assumed the resistor value was not critical and used 4k7 as that was what I'd used for the I²C pullups. Looks like the contrast pin can supply ~300uA (1.5/4700) to charge the capacitor. Another lesson learned.

Mike.

 

[Pommie]

OK, soldering a 1206 resistor onto the existing components is turning out to be a problem, ruined one board already. I'm wondering if a 1k resistor across the capacitor would work? In effect there will be a voltage divider consisting of a 4.7k to I/O pin and a 1k to ground with a 10uF capacitor from the junction to ground and the junction going to the contrast pin. Assuming the contrast pin can source 300uA, I think I should be able to go down to about 1/4V (824*0.0003) - the 824 being the 4.7k and 1k in parallel - I/O pin at 0V. Is this a reasonable assumption?

Thanks,

Mike.
Edit, To solder a resistor across the cap is simply from the contrast pin to ground. So nice big connections.

 

[Nigel Goodwin]

I see from your picture you're using a 4 line LCD, have you tried your module on a 2 line one? - while I've got some 4 line LCD's the ones I've used 1K and 10uF on have all been 2 line ones.

As the 4 line ones tend to be two 2 line ones added together, it 'might' be doubling the current available from the contrast pin?.

 

[Pommie]

What is a 2 line LCD?

Mike.

 

[Nigel Goodwin]

What is a 2 line LCD?

Mike.

2 lines of 16 or 20 characters (or even 8 or 40 characters) - most LCD's are two line.

I'm presuming yours is 4 lines of 20 characters?.

What is a 2 line LCD?

Mike.