RC transistor circuit?

[MrDEB]

Got brain freeze tis morn.
I have seen this circuit and simulated in LT SPICE but just can't duplicate.
the transistor is supposed to stay on for 10 seconds.
I don't think I have the right combo or right component layout
yes well aware that its a very basic circuit but can't think straight today??
this got e bugged last night.

 

[ericgibbs]

hi Deb.
That 1K across the 220uF will not give anywhere 20secs.

Post your *.asc file.

 

[MrDEB]

its a 1 k resistor but tried several different resistors.
for sure doing something wrong but ??
will try and post asc file

 

[BrownOut]

The transistor is probably in saturation, and so base current it very high, quickly discharging the cap. Try a timer circuit based on a 555 instead.

 

[MrDEB]

this is just bothering me

have seen this circuit on the web but?
perhaps the base resistor connected directly to base to limit cap discharge.
presently the led stays on same as pulse.

 

[BrownOut]

Without a resistor to limit current into the transistor, you're just discharging the cap into a forward bisaed diode.

 

[ericgibbs]

hi Deb,
When you set the pulse back to 0V from +5V in your Vsource you are discharging the 220uF back thru the series 1K.

 

[MrDEB]

I relize that the cap is discharging to fast

I am just expereminting with different configs.
It bothers me that I can't locate the web site that had similar but using a relay instead of LED.
had resistor/capacitior combo to turn on transistor for 10 seconds.
Added resistor to emter, increased cap and base resistor but not what I recall on web site that I can't locate.

 

[Rolf Zetterberg]

This?
10 Second Fan ON delay time by transistor | Circuit Project Electronic

 

[BrownOut]

I think that's a delay on "make" He wants a delay on "break"

 

[ericgibbs]

hi deb,
Look at this version.

 

[MrDEB]

I am just curious as I know I saw a website that had the cap and resistor in parallel connected to a transistor.
going on a mission to locate that site. it bugs me I can't find it.
QUESTION = where did you get the relay for LT spice?

 

[MrDEB]

Eureka i found it!!!

http://www.reuk.co.uk/PIR-Sensors.htm

Here is the circuit I was looking for.
Going to try in LT SPICE just out of curiosity.

 

[crutschow]

That website circuit would only work if the transistor were a FET.

 

[Grossel]

That website circuit would only work if the transistor were a FET.

That's sure.

The chematic as it is will alow the cap voltage to reach (values is just plausible guess) 0.75v while cap voltage is 0.55V when it cease to conduct. Also the base current for a given voltage value is almost impossible to calculate accurate because transistors (even same type) have different characteristcs.

 

[Sceadwian]

Use a Schmitt triggered inverter, without the schimit trigger you're going to struggle avoiding the linear region of the transistor. Schmit triggers can EASILY be setup to produce one shot values based on an RC time constant with feedback.

**broken link removed**

 

[BrownOut]

That circuit has the same problem as your original one. Whomever put that up should be stripped of his soldiering iron.

 

[ericgibbs]

QUESTION = where did you get the relay for LT spice?

hi,
Goto this site and sign up, there are lots of examples.

**broken link removed**