transistors

Stompa

I don't see how the datasheet I linked to could be wrong. Which would imply that the numbers on the circuit diagram either:

(a) are simply incorrect.
(b) refer to a differently packaged variant (i.e. non-TO92) of the same transistor which may have different pin connections.

I'd have thought it fairly unusual to include the pin numbers on the circuit diagram (unless the package type is also specified) - though having said that my knowledge of this sort of stuff is about 25-30 years out of date!

espresso

Wig wrote: »

I
How do I test for the base?
Is there no test for emitter & collector?

Forget the 1,2,3 it is EBC or CBE that matters.

It is explained in the link in post #15 how to test for an unknown bipolar transistor, assuming that you have a multimeter.

Wig

bookduck wrote: »

Since you are unsure what you are doing, you could have got the ready made thing for about £16 inc p&p via ebay. And it has all the right connectors too

http://cgi.ebay.co.uk/VW-OBD2-RS232-Diagnostic-Scanner-Interface-Tool-Audi-A3_W0QQitemZ290236861265QQcmdZViewItem?IMSfp=TL0806070914a11693

If I was to buy one, I'd want one which accessed all protocols eg ISO, VPW, PWM & CAN. And those cost about £23 - £25 within UK, and £15 from HK & China. The Ebay listing seems to have some confusion about USB, it says it contains a USB to RS232 chip which makes no sense at all. But for most European cars the one linked to on Ebay would be ok as most are ISO, and it is quite cheap at £17.

However, my DIY effort will cost me about £1 using old bits I have lying around. But if anyone was to buy everything it would cost a maximum of £6 inc an OBD plug. I might very well buy one from Hong Kong, and may also build my own good quality one using an original ELM327, and compare them all to see any differences.

Plus the big advantage of doing it yourself is that you learn a lot more about the subject and the hardware. And I have now learnt how to identify a transistors pin outs.

I got the following results with multimeter set to 'diode' and pins read as 123 left to right with flat side up.

BC546
1+ 2- OL
1- 2+ 805
1+ 3- OL
1- 3+ OL
2+ 3- 815
2- 3+ OL

Which means the base is 2 and + (P) so NPN. And the higher voltage is on 3 so 3 is the emitter

BC556
1+ 2- 828
1- 2+ OL
1+ 3- OL
1- 3+ OL
2+ 3- OL
2- 3+ 834

Base = 2 - (N) =PNP
higher = 3 = emitter

so both are confirmed to be the same as the data sheets, i.e.


My multimeter also has a transistor checker and the hFE for both came out as 314 which is apparently the DC current gain. The data sheets said this value should be between 100 - 800.

Now I can crack on and build it.