Maintaining transistor bias

Thread Starter

hondabones

Joined Sep 29, 2009
123
What are some examples of keeping a transistor biased after it is triggered by a sensor? I have designed a motion sensing circuit that when movement occurs it bias' an npn transistor. After movement stops the transistor turns off after about 4 seconds. I would like to at least double that time.
 
Last edited:

yourownfree

Joined Jul 16, 2008
93
I am having trouble seeing what is happening. I see a movement causes the transistor to turn on. Then no movement turns it off ? Then you say you want it to stay off longer? Is this before it sees movement again? What I mean is do you want it to stay off longer even when it sees movement? Then after set time work again to see movement?
 

Thread Starter

hondabones

Joined Sep 29, 2009
123
I think you misread my post. Movement causes the transistor to turn on. After movement stops the transistor remains on for approximately 4 seconds, then turn off. I would like to extend the time it takes before it turns off.
 

Adjuster

Joined Dec 26, 2010
2,148
Whether adding a capacitor (and perhaps a diode and/or some resistors) to the base circuit could achieve what the OP wants would depend on what levels of output voltage and current the PIR sensor can provide.

Without having more information, it's not clear whether you could add an extra four seconds in this way without excessively reducing the sensitivity to turning on in the first place. The timing would not be precise, and the switch-off would be gradual.

Adding a timer circuit like the 555 or generally some kind of monostable would use more parts, but the result would be more predictable.
 

Thread Starter

hondabones

Joined Sep 29, 2009
123
I have tried using capacitors, however the voltage from the sensor isn't "hearty" enough to charge anything. I was trying to avoid using a 555 timer because that would mean rebuilding and redesigning. I would have all this fixed if I could just get my hands on the PIR sensor schematic. Then I could just change the timing resistor on the sensor. Looks like I am going to have to redesign this project. Thank you to all for your replies.
 

Thread Starter

hondabones

Joined Sep 29, 2009
123
I refreshed the page and I can no longer see Voodoo's post however I did catch a glimpse of it and added a thanks. I also like Bertus' idea. I was flirting with this type of idea already. thank you both. When I get a chance I will try these out.
 
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