555 timer trigger question

Søren

Joined Sep 2, 2006
472
Hi,

I only said 555 because I figured it needed some type of timed relay and a 555 timer is cheap.
OK, then you have a couple of circuits to choose from.


I wanted to know if a 100nf is == 0.01 uf.

and if BC547 == 2N3904

I know nothing about transistors so I tried matching everything up as close as I could. emitter-base is 6v and max volts is 60v, those were the same I though they'd work the same. Can I get a confirmation?
100nF is 0.1µF
0.01µF is 10nF
However, it's not critical, so use 0.1µF or 0.01µF, whatever you've got.

The BC547 can be replaced by just about any small signal NPN. a 2N3904 is quite fine.
 

Thread Starter

skinner927

Joined Dec 31, 2007
36
Beware of programmers who carry screwdrivers.
100 nF == 0.100 uF



Both the BC547 and 2N3904 are General Purpose NPN transistors.The BC547 is rated for higher switching currents (300 mA) than the 2N3904 (100 mA). Either will do in this circuit as R3 == 100 kOhms.

Regards.
Thanks, that saved the day!

Funny... I am a programmer. haha, I guess the == gives it away.

and just to confirm,
470 nF == 0.47 uF?
 

Audioguru

Joined Dec 20, 2007
11,248
Both the BC547 and 2N3904 are General Purpose NPN transistors.The BC547 is rated for higher switching currents (300 mA) than the 2N3904 (100 mA).
My datasheets have the BC547 with a max allowed collector current of only 100mA (the current gain drops above 40ma) and the 2N3904 has a max allowed collector current of 200mA (its current gain also drops above 40mA).
 

jvjtech

Joined Jan 26, 2008
23
I looked up Fairchild datasheets for both the BC547 and the 2N3904. The following text is from these datasheets:
For the BC547: This device is designed for use as general purpose amplifiers and switches requiring collector currents to 300 mA.
Absolute maximum: IC Collector Current - Continuous 500 mA
For the 2N3904 This device is designed as a general purpose amplifier and switch.
The useful dynamic range extends to 100 mA as a switch and to 100 MHz as an amplifier.
Absolute maximum: IC Collector Current - Continuous 200 mA
I did not look through the datasheets for other differences which may come from operation as an amplifier as compared to operation as a switch. For the latter, the device is saturated and may have a higher allowed collector current due to the lower Vce. In this application the device is operating as a switch so I used these data. In any case, as can be seen, a study of the datasheets for the device one uses is in order. Regards.
 

jvjtech

Joined Jan 26, 2008
23
and just to confirm,
470 nF == 0.47 uF?
There is a saying about giving a man a fish as opposed to teaching him to fish.

Capacitors come in values separated by multipliers in 1000s as follows:
1 uF (microfarad) == 1 x 10-6 F
1 nF (nanofarad) == 1 x 10-9 F
1 pF (picofarad) == 1 x 10-12 F

And yes: 470 nF == 0.470 uF
Note that I added the trailing zero to help keep track of the 1000s.
Another memory mnemonic is that the nano in nanofarad sounds like nine, the 10s exponent for this value, i.e. 1 x 10-9.
Regards.
 

Thread Starter

skinner927

Joined Dec 31, 2007
36
There is a saying about giving a man a fish as opposed to teaching him to fish.
Thank you, I've written all that down now.

Just a question about the transistor now, is the 2N3904 a yes or no for an acceptable replacement for the BC547? If it were cars or computers (things I fully understand) I would greatly appreciate the explanations alone, but a simple yes/no would be perfect if it were included in the answer.

Not to say that I don't appreciate the explanations but at this time the detail going into the answers have nearly no meaning to me.

Again, thank you soooo much to everyone that has helped me out.

oh, and does anyone know what a circuit like this would be called?
 

Thread Starter

skinner927

Joined Dec 31, 2007
36
I'd like to thank everyone who has helped me on this. I realized that a 555 timer is a little overkill for a one-shot type of thing.

I designed this circuit, and I was wondering if anyone could give me a confirmation that it would work, at least if it's plausible.

Please, feel free to bash away, the only way to learn is to learn from your mistakes :D



All diodes are 1N4002, Transistor is a BC548B, and relays have a 6v coil.

thanks for the help, i'm very very new to this, as you know.

The 5V circuit at the bottom right is only so I know when the relay is tripped, again, for testing only.
 
Søren;53570 said:
Hi,



This should do then:

When turning the ignition on, Q1 pulls C2 down triggering the 555 and turning the ignition back off pulls C1 down triggering the 555 once more. Should it ever "get out of synch" due to fiddling with the ignition (could happen although highly unlikely), just press the button on the radio once manually.
D1 and D2 keeps C1 and C2 from swamping each other.

A CMOS (LMC)555 is specified to keep quiscent drain down, since it has to be on at all times. An ordinary LM555 would drain around 7.5 to 15mA, while the LMC555 only takes around 1/50 of that.

No values are too critical, C1 and C2 could be anything down to around 1nF if needs be.


Personally, I would find a way to emulate the behaviour of the ECU towards the radio, insted of having to wire into the button on the front of the radio - couldn't be that hard after all.


This is the circuit I have been searching high, and low (no pun intended) for!!! I pretty much had it all done, except the blocking diodes D1, D2, so that was the big problem there. Would you call this a dual triggered one shot?

Thanks so much!!!
 
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