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easy basic stamp question

M

Mike_in_SD

Jan 1, 1970
0
Having a brain burb here.

I need a basic stamp (actually a basic atom) to know when I step on the
brake pedal of my car.

I have to change the 12v into a 5v/high for the stamp.

Using a 7805 seems like way overkill

thanks
mike
 
H

Homer J Simpson

Jan 1, 1970
0
Mike_in_SD said:
Having a brain burb here.

I need a basic stamp (actually a basic atom) to know when I step on the
brake pedal of my car.

I have to change the 12v into a 5v/high for the stamp.

Series resistor and zener.


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M

Mike_in_SD

Jan 1, 1970
0
[email protected] (Homer J Simpson) wrote in
Overkill for a sense input.

My ohms law isnt totally gone .. but is it

R = 12v_source/30ma sinking capacity of stamp pin

or

R = 7v_needed to get rid of/30ma sinking capacity of stamp pin

and what about the zener ?

sorry about the hand holding .. but .. this is "basic electro" grin


mike
 
H

Homer J Simpson

Jan 1, 1970
0
My ohms law isnt totally gone .. but is it

R = 12v_source/30ma sinking capacity of stamp pin

or

R = 7v_needed to get rid of/30ma sinking capacity of stamp pin

and what about the zener ?

sorry about the hand holding .. but .. this is "basic electro" grin

Use the lowest current you can reasonably find and a resistor that is enough
to turn it on. You don't need to dump max current into the pin for input.


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F

feebo

Jan 1, 1970
0
Having a brain burb here.

I need a basic stamp (actually a basic atom) to know when I step on the
brake pedal of my car.

I have to change the 12v into a 5v/high for the stamp.

Using a 7805 seems like way overkill

thanks
mike

it depends on your car's system... some have 12v at the switch and
when the switch closes, the "output" of the switch puts the 12v on the
wire to the brake lights. If this is the case, see the following link

http://www.luddite.me.uk/brakeswitch/image1.gif

this gives you a HI on the output when 12V appears at the input. This
is OK, but you might like to replace the zener set-up with an
opto-coupler just to be doubly safe - only about $0.50 for a suitable
6 pin device

Alternatively...

if your car has 12V on the light cluster and when you press the brake
the switch provides a link to ground, see this link

http://www.luddite.me.uk/brakeswitch/image2.gif

this gives you a LO when you press the brake (deal with the ac tive LO
in software).

both circuits will provide a jittery output as no debouncing is done -
that is the signal will not switch cleanly but have some spikes when
you first press and then release the pedal - this could be sorted with
small capacitor between the outupt to the PI and ground
 
M

Mike_in_SD

Jan 1, 1970
0
feebo wrote in said:
wire to the brake lights. If this is the case, see the following link

http://www.luddite.me.uk/brakeswitch/image1.gif

the switch provides a link to ground, see this link

http://www.luddite.me.uk/brakeswitch/image2.gif

this gives you a LO when you press the brake (deal with the ac tive LO
in software).

both circuits will provide a jittery output as no debouncing is done -

Im using it as an interrupt so jittery is ok ...

thanks so much for the diagrams ...

mike
 
F

feebo

Jan 1, 1970
0
Im using it as an interrupt so jittery is ok ...

thanks so much for the diagrams ...

mike

hmmm... I suspect the non-debounced output will cause multiple
triggering of your interrupt - is this OK?
 
M

Mike_in_SD

Jan 1, 1970
0
feebo wrote in said:
hmmm... I suspect the non-debounced output will cause multiple
triggering of your interrupt - is this OK?

yea .. Im pretty sure it will be ok.

What I am doing is ... I put a modern a500 automatic tranny in my
34 ply street rod. Normally this tranny is computer controled as to
when it shifts into overdrive and locks up the torque converter etc.

So I am building a controller to control when it shifts etc, and when
it senses that I touched the brake, it should disingauge the converter
immediately ... so I dont care if it bounces.

they should make a 7805-5 where you have 5 7805's all on one chip
... for just this scenario, where you just want to do a ttl level
"sense" of the 12v.

anyways .. thanks for the help
mike
 
W

Wim Lewis

Jan 1, 1970
0
[email protected] (Homer J Simpson) wrote in


My ohms law isnt totally gone .. but is it
R = 12v_source/30ma sinking capacity of stamp pin

or

R = 7v_needed to get rid of/30ma sinking capacity of stamp pin
and what about the zener ?

That would be what you would want if the stamp pin were acting as an
output, and you wanted to avoid burning it out. But that's not
what you're doing. Instead you're using it as an input. You don't
really care how much current flows, as long as the stamp can sense
the voltage correctly and nothing gets damaged.

There's a common trick for this. All you need is a largish resistor.
You don't actually need a zener or anything remotely as complex as
a 7805. I'll explain.

Logic inputs normally don't let any current flow (they're just
voltage-sensitive) but they have a pair of "protection diodes" on
the chip, going to the positive and negative (ground) rails, to
keep the input voltage from going out of bounds and damaging the
actual logic gate. The protection diodes aren't designed to take
very much current, but they can take some. So, let's say you simply
connect your stamp input to the 12v signal through a resistor, which
I will randomly choose a value for, of 100K. Some current will flow
through the resistor, into the input pin, through the protection
diode, and onto the +5v supply. How much current? Well, (12v - 5v)/100K,
or 0.07 milliamps. Not very much! (Actually, it will be a little
less, since the protection diode will have some voltage drop
associated with it.) 0.07 mA is small enough that it won't hurt
anything to have that much current going through the protection
diodes. So this ought to work fine.

Going into more detail, though, what values of resistor are OK?
There'll be a lower limit and an upper limit. If the resistor is
too small, then too much current will flow through the protection
diodes (which, remember, really aren't intended to carry very much
current) and burn them out, probably followed by burning out the
rest of the chip. If you look in the datasheet for the PIC on the
Stamp, in the "Electrical Characteristics" section it lists the
maximum "input clamp current". The particular PIC I just looked at
says it can handle 20mA there. (That's more than I thought!)
I'd be conservative and limit it to 2 mA, which works out to a resistor
value of (12v - 5v)/2mA = 3.5K. So any resistor larger than 3.5K
should be safe.

What's the upper bound? How large of a resistor can you use before something
stops working right? This is mostly determined by the capacitance of the
input pin, and its leakage current. The capacitance will bite us first
so I'll just calculate based on that. The datasheet says the IO pins
have at most 50 pF of capacitance. When the signal voltage changes, that
50 pF has to charge or discharge through the resistor. To a *very* rough
approximation, the time it takes for that to happen is R*C, the resistance
times the capacitance (if you work out all the units, you'll find that
ohms times farads equals seconds, so this makes sense). If you make the
resistor too large, then the input won't see changes quickly enough.
For the 100K resistor I mentioned earlier, R*C = 100K * 50p = 5 microseconds.
That's much faster than you need if you're just sensing a brake pedal.
In fact, you might want to hang an extra capacitor onto the input pin
(after the resistor) to slow down the input to (say) 5 millisconds, which
could also filter out small glitches and spikes on the line and help
to "debounce" the input a little.


I hope this explanation helps a little.
 
F

feebo

Jan 1, 1970
0
yea .. Im pretty sure it will be ok.

What I am doing is ... I put a modern a500 automatic tranny in my
34 ply street rod. Normally this tranny is computer controled as to
when it shifts into overdrive and locks up the torque converter etc.

So I am building a controller to control when it shifts etc, and when
it senses that I touched the brake, it should disingauge the converter
immediately ... so I dont care if it bounces.

they should make a 7805-5 where you have 5 7805's all on one chip
.. for just this scenario, where you just want to do a ttl level
"sense" of the 12v.

anyways .. thanks for the help
mike

but the 7805 is a voltage regulator - it's designed to give steady
power - dunno how it will like being switched. just use an interface
circuit similar to those I posted - gotta be cheaper too.
 
R

Rich Grise

Jan 1, 1970
0
Overkill for a sense input.

What overkll? It gets the job done in one part. Have you ever seen
an L05? They come in TO-92 (or something very much like that.)

Cheers!
Rich
 
B

Byron A Jeff

Jan 1, 1970
0
Having a brain burb here.

I need a basic stamp (actually a basic atom) to know when I step on the
brake pedal of my car.

I have to change the 12v into a 5v/high for the stamp.

Using a 7805 seems like way overkill

Probably not enough. The Stamp is expensive enough that you'd want to
protect it from the harsh electrical environment of the typical auto.

Use the 7805. Then at the very least put a current limiting resistor and
a hefty zener diode in front of it.

Think about the fact that car electrical systems can load dump and voltages
of upwards of 60V can be placed on the system, frying everything in sight.

Be prepared to sacrifice a dime zener or a 50 cent 7805 instead of the
pricey stamp.

BAJ
 
M

Mike_in_SD

Jan 1, 1970
0
feebo wrote in said:
but the 7805 is a voltage regulator - it's designed to give steady
power - dunno how it will like being switched. just use an interface
circuit similar to those I posted - gotta be cheaper too.

Im thinking you may be right feebo. The speed sender sends pulses that
I have to interpret ... so I havent looked at the pulses under a scope
yet .. but .. I would think that it will be 12v switched very fast so
I really doubt that the 7805 would work as a signal conditioner to
the pic. your diagram is probably the ticket.

thanks again
mike
 
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