Chris,
Depends a bit on what current is drawn by the relay coil.
A typical small PCB mounted relay would draw about 13 mA, so
to ensure that Q1 is in saturation, its base current should
be about 1.3 mA (1/10 of the collector current is a good
rule of thumb for saturated switching transistors) this
gives R7 = 12/ 1.3 kilohms approximately.
i.e. R7 = 9.2 K Probably use R7 = 10 K, as an easily
obtainable value, (or 8.2 K if you are nervous !).
(The relay I based the above on is a Fujitsu type FBR46 with
12v coil, available from Radio Parts here in Victoria,
Australia)

Would Peter Nyffeler care to comment ?

Cheers, John G.

"Chris" wrote in message
. au...
| Looks like a good project, what should the value of R7 be
?
|
| Chris Runeckles
| Western Australia
|
|
| "Roger Druce" wrote in message
| u...
| Perhaps someone out in r.a.s land may be able to point
me in the direction
| of a commercial piece of equipment which would perform
the following task.
|
| I would like to be able to charge up a 12 volt (nominal)
gell cell battery
| and then plug it into a piece of equipment to check its
total AmpHour
| capacity during discharge at some constant rate current
(say the "10 hour"
| rate or the "20 hour" rate). All this towards assessing
whether the
| battery
| has proven useful capacity and life left in it, or has
it reached its
| discard date?
|
| The equipment would show the delivered amp-hours up to
the point at which
| the voltage has fallen to a nominated end voltage.
|
| I realise that you can do it in a way with a light globe
to discharge the
| battery and then time the length the light holds up
glowing brightly.
| This
| is all too primative for this day and age, and there
ought to be some
| piece
| of equipment available for the task.
|
| Or some circuit from an electronics hobby magazine??
|
| Thanks in antcipation.
| Roger Druce
|
|
|
|