DIY injector cleaning

[Tom Cloud]

>>   bruce,
>> 
>> I like the circuit and we will try it at the shop.
>
>One change to the circuit:
>
>  Drive the 555 timer with +5 volts instead of +12, because the
>  TIP120 transistor VBE rating will be exceeded (according to
>  the back of the blister pack from Radio Shack). Still drive
>  the collector of the TIP120 with +12 volts (the injector
>  circuit). You can get +5 volts simply from a 7805 voltage
>  regulator (also at Radio Shack).

Bruce, it is with great trepidation that I venture in here ... may
I disagree with your above statement ...

 there's several things at work here.  A BJT (i.e. the TIP120) operates
on current (IB) and the 330 ohm resistor converts the drive voltage
from the 555 into a base current.  If the voltage is less -- or
greater -- the resistor would need to be adjusted.  Adjusted to
keep the current draw from the 555 within it's ability to supply
it, and to supply enough to insure switching from the xstor.

then, there's VBE and VEB.  VBE is the BE diode forward voltage.
It's "typically" .7 volts, though it'll more than likely be closer
to 1 volt.  (If the 330 ohm resistor were not present, the 555 would
be essentially driving a 1 volt short circuit.)  Then, VEB.  This
is the BE diode reverse voltage.  Not looking at the TIP120 data,
but I can tell you that few BJT's have over 6 to 10 volts reverse
voltage rating.  But, this circuit has no negative voltage, so the
VEB rating should not be exceeded (he'd have to worry if he had
a speedup cap or an inductor in the base circuit).

One last question -- why doesn't he use a switching, logic MOSFET ?