🚀 go-pugleaf

DaveC <anon@example.net> wrote:

>Using a small gp transistor, I think I can eliminate the relay altogether:
>
>Piezo "buzzer" 3-12v, (10mA @12V), 90dB
>gp NPN transistor, 200mA max. cont. Ic. 
>R=1Mohm, C=0.1uF (this gives T=10 seconds, yes?)

No, that has a time constant of 0.1 sec, from
T = RC = (10^6) * (10^(-7))

Also, you need a time constant much shorter than 10 s, because of your
circuit configuration. (See recommendation below.)

>9V zener
>2 gp silicon diodes 
>
>Buzzer placed between +12v and collector. Diodes from emitter to each sensor 
>terminal. (These terminals will each be 12v until a sensor grounds when 
>pressure goes low, or temp goes high.) R-C connected between GND and +12v (R 
>"on top"). Zener from R-C junction to base. 
>
>I now ask:
>1) Is it OK to have a current path from +12v through the buzzer, through the 
>transistor, through the diode, to ground?

Yes, but buzzer will be a little quieter than max, as it's operating
below its rated 12V because of the diode drop (and small CE drop).

>2) Is it OK to have a 1Mohm resistor providing base bias via zener. Should I 
>choose a different RC combination such that R is smaller than 1Mohm?

Yes, you should - much smaller. With 1M you won't get anything like
sufficient base current.

I suggest R = 4.7k and C = 680uF, assuming you're using a 9.1 V zener.

>3) What type of cap? (ie, poly, etc.)

Good quality electrolytic. Obviously, turn-on won't be crisp from such
a crude circuit, but it might do the job.

>This circuit will be under the dash, so max. ambient temps of 150F (hot days, 
>windows closed) should be expected. 

I'd breadboard it and try it first in some similarly hot environments,
like my oven!

Terry Pinnell
Hobbyist, West Sussex, UK