hai
im newbie here..erm..ive done assembling my own digital tachometer circuit using lm2917..when i tested i found that the led wont lit up unless i touched the resistor (470ohm) using my finger.. i wonder why this is happen..plz help me find solution..

 

Did you find this circuit somewhere online, or did you design it yourself?

If you found it online, please post a link to where you found it.

I don't see your LED in the schematic.

Your 470 Ohm resistor connection may not be good.
The LM2917 has a built-in 7.56v Zener diode that regulates the voltage on pin 9. The 470 Ohm resistor should be connected to a 12v source, not an 8v source.

I don't see a capacitor on the output of the regulator, nor does the schematic show what the regulator is. Regulator ICs need an input and output capacitor, or they may oscillate at high frequencies (in the 2MHz to 10MHz range).

 

actually i found this cct online. this is the link :
http://foz11.tripod.com/DIY_MOD_s/Digital_Speedo/digital_speedo.html

this cct is only 1 of the circuit..this cct will convert pulses to voltage than the other cct which consist of lm3914 that will lit the led according the voltage generated at coil..the other cct is shown below



so, i only have to connect the 470 resistor with 12V supply leaving the rest using 8 v supply? is it like that? and adding capacitor at the output..okey i'll work on it...tq