bc557 to 2n3906

[TrickyNekro]

Hello guys,
I'm building a rev meter for my ride, and I found a schematic doing the preamp of the tachometer signal, with a
BC557..... I looked at the datasheets..... they pretty much look the same with 2n3096 being more
sensitive and having a higher gain, Still.... I'll be only counting pulses with an ATMEL..... so....
I dunno whether I have a problem.... Still.... I kinda worry about my car..... And I have to ask


Best regards, Lefteris
Greece


PS I'll try to upload some schematics ASAP

[Soeren]

Hi,

Even without seeing the schematic, I can't see any reason for not making the substitute. It's hard to imagine that it should be able to damage your car - at most (if it's a poor design), it could refuse to work, but any designer worth his work will never use a transistor to it's limits, but will stay within the worst case tolerances (at least).

If you post the schematic, I'll tell you for sure whether there's any problems with the swap.

[TrickyNekro]

Well.... I too guess that I would have no problem and it's just to drive the 74HC221 following
So I posted the schematics, which are part of an Elektor project.
The original Elektor project doesn't use a microcontroller which makes the schematics less portable....
So I replaced the ADC and the PWM ladder style system with a micro counting pulses....
And I included the 74HC221 to adjust T1 over T2.... wanting a duty of 50%.....

Also another question.... 74HC221 is quite old.... But I managed to found an 74LS221....
Is that of any difference.... The only deference I noticed was the Schmitt trigger like output that LS had.... but really didn't noticed
if that was featured on HC too...... Still I got to test the circuit with a NE555 and a multimeter....
Ready I do hope I get some decent readings....


Thanks for the help,
Best regards, Lefteris
Greece

[Soeren]

Hi,

Sorry for the delay. I'm on vacation with cellular broadband which is acting up due to the topoloy of the area (we're practically behind a shield of sand and forest, seen from the nearest antenna) and this is the third time I try to post your answer, so this time I'm finally getting smart and writing it offline first

The BC557 is fine for your app.

Also another question.... 74HC221 is quite old.... But I managed to found an 74LS221....
Is that of any difference.... The only deference I noticed was the Schmitt trigger like output that LS had.... but really didn't noticed
if that was featured on HC too......

The HC221 has got Schmitt triggers on the B-inputs (pins 2 and 10)
The LS is much older (and craves more current) than the HC, but probably is more available as well btw.

Did you consider the LM2907/LM2917 for your app?
You could do worse than reading the datasheet and the app note here:
www.national.com/ds/LM/LM2907.pdf
www.national.com/an/AN/AN-162.pdf


Yet another method would be to just clean up the pulses if you're using a very old (non-EFI) car or take an output from the ECU if it has got one and then time the _period_ of the pulses, average 3 or more periods (or rather the reciprocal of the periods) and you should be set even without using A/D-C.

[TrickyNekro]

Man, couldn'r be worse than a dial up on a 7 year old laptop.... You aren't the only one on holidays!!! :-P

Thanks a lot for the answer, true is that the original circuit is using an ADC to do the conversion still....
I prefer the pulse way, using a timer interrupt based program (isn't this called a close loop program???)


Have some time relaxing and thanks again for the answer!!!

Best regards,
Lefteris, Greece



edit: I found both HC and LS so that wouldn't be a problem anymore!!!!
When I get confident with my car I'll try the other methods too!!!