relaly driver latching

[Starbase]

Hi all,

My electronics is not good but I am trying to build a circuit which will
monitor battery voltage (the battery is charged from a solar panel) and
switch a relay on when it reaches a set value (say 13.5 volts) and off again
when it falls below another value (say 11.5 volts). I built a voltage
monitor with LEDs based on the LM3914N and that works OK. Also to conserve
power I power it in short bursts through a circuit driven by a 7555 timer
and that works OK. Now I am working on the relay driver which will be
switched on from a positive pulse from the LM3914N on a selected pin feeding
one of the high voltage indicating LEDs and switched off again on a positive
pulse from one of the low voltage indicating LEDs. My theory was that I
build a simple relay driver which switches the relay on and is then self
sustaining from a lead with resistors from the emitter feeding back into the
base, the coil load being connected between ground and the emitter of the
driving transistor. Hopefully this would keep the relay latched and to
unlatch it a second transistor would be used to sink the current feeding the
base of the first transistor to ground. I am still working on this part of
the circuit (using unknown transistors from an old TV with markings "JC 501Q
230", I have established that the transistors are NPN with a hFE of about
260 and though I can get one to switch a load (I am currently using an LED
as the load) I still have some experimenting to do to get it to latch, i.e.
it does not work yet.

I just wondered if this was a standard way of doing things, what you think
of it, or are there any other (SIMPLE) approaches that I should consider
which will work.


Regards Chris


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[Anthony Fremont]

Hi all,

<snipped description of very power hungry circuit

I just wondered if this was a standard way of doing things, what you think
of it, or are there any other (SIMPLE) approaches that I should consider
which will work.

MOSFET's and PICs will be your best bet IMO. You would then be able to
cut the average required power to well below 1mA.

 

[Fritz Oppliger]

I still have some experimenting to do to get it to latch,

You need a flipflop, either use a chip or implement one with your discrete
parts... which is more punishment than you deserve.

 

[Starbase]

You need a flipflop, either use a chip or implement one with your discrete
parts... which is more punishment than you deserve.

I think I will experiment with this idea first (above), though I do much
like the idea of using and programing a PIC, a leap I am almost ready to
make.

Many thanks for the ideas chaps.

 

[Anthony Fremont]

"Starbase" wrote

I think I will experiment with this idea first (above), though I do much
like the idea of using and programing a PIC, a leap I am almost ready to
make.

It's one leap in life that you will never regret.

 

[Rheilly Phoull]

"Starbase" wrote


It's one leap in life that you will never regret.

OTOH just a simple comparator chip directly driving a power mosfet is the
'conventional' approach and has very little current overhead ( 5mA tops).