Help me solve a bug in txr of my robot

[electron_boy]

bug in my robot

hi everybody,
last month for an exhibition in our college i did my first bot which can sense any obstacles and can turn its path. for detecting obstacles i constructed a proximity sensor with ir txr and rxr(tsop1738)

i designed the txr with 555 astable for 38khz.but practically i got only around 25khz then i modified it putting a pot instead of 'Ra' and measured the frequency as 38khz but the theoretical value was aroun 50khz what would be probably wrong.
plz help

bye

 

[nicleo]

bug in my robot

The external capacitor charges through RA + RB and discharges through RB.

The error is approximately 30%.

Error source could be:
1) Charging/Discharging capacitor
2) Internal ESR of Charging/Discharging capacitor
3) RB resistor

 

[IanP]

Re: bug in my robot

I wouldn't blame resistors unless you used something unknown, not for error of this sort..
The most probable reason will be the capacitor as caps quite often have +/-20% (or more) tolerance..

 

[nicleo]

Re: bug in my robot

I wouldn't blame resistors unless you used something unknown, not for error of this sort..
The most probable reason will be the capacitor as caps quite often have +/-20% (or more) tolerance..

I agree with you, IanP. The error due to RB and probably the ESR in the cap was compensated by the POT already. Should be the cap in this case.