A £1 Lcd?

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Peter-C

Member
I picked up an electronic exercise step counter in my local Pound Shop this week (for £1). It has an LCD display of 5 digits and I thought I might be able to use it to display a given variable value from a picaxe.
Sure enough, it is designed to increment the count each time a contact is made, using an internal sprung pendulum. It also has a button to zero the count.
I have wired connections to it, that work as hoped, in that I can zero it by touching one set of wires together and display any variable from a picaxe using a for-next loop to count up to the number I want displayed.
So far so good, a very basic LCD for a pound.
I can successfully drive the counting process by placing resistors from 5V to the device connections and back to output 1, with this code:
readtemp 1, b3
for b0=1 to b3
high 1
pause 120
low 1
next b0
Unfortunately the connections to zero the count are more fussy and seem to malfunction if they pick up any voltage from the Picaxe circuit.
Obviously I could use a relay to give a make/break connection, to operate the device's zeroing function, but I wonder if there is a better way of providing a simple make/break switch function, driven by picaxe, that doesn't leak any power to the counter.
The ability to display variables such as temperatures on a very cheap self powered display, using only one or two output pins would be very attractive to me. Can anyone help me with the electronic options for the make/break switch electronics?
If so I'll go and clear the pound shop of its existing stocks!
 

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eclectic

Moderator
#Riccardo.
What you find in any pound shop is completely down to luck.


Otherwise, search for Manuka's old thread
on using a cheap bike speedometer.

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Dippy

Moderator
My only experience of totaliser counters has shown that the reset/zeroing is done by taking the reset pin to ground briefly.
Get a multimeter and see if one of the pins (to the P/B switch) is grounded.
The measure the other one and see what the voltage is.
Then, powered off, check to see if the grounded one is connected to ground with a continuity/resistance test.
(assuming no resistors involved).
Or if its the reverse, then do the reverse.

1. What is the voltage supply to the counter?
2. Are you conencting to a PICAXE at the same voltage?

If you are chucking in 5V from PICAXE when the LCD is 3V you could break it !!!!


But without the details of that type I can't say 100%.


If the reset button is simply connecting a pin to ground then you should easily be able to reset/zero from PICAXE, but beware the voltages.
If necessary, you can use a res+NPNtran to behave like a switch to ground.
Even a level shift may be OK...
BUT, YOU must test the pins to confirm/check this FIRST.
It should be a piece of cake.

I assume rmeldo isn't going to drive from Brighton to Bolton to save a couple of quid? ;)

This tatty Data Sheet may give you some guidance:
http://www.farnell.com/datasheets/21019.pdf

NOTE: It is for a different totaliser, but may show what I'm trying to explain badly.
 
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BeanieBots

Moderator
I've used several of these little modules for battery capacity meters.
I tried an assortment of driving methods but like you, found the reset to be very fussy and sensitive.
The best solution I found was to use a 4k7 and 10nF in series between PICAXE output and module with common ground. Then use pulsout for counting and resetting.
The R limits current and the cap avoids any continuous current due to voltage level differences. The 'pulse' has both rising and falling edges so no need to worry about edge polarity. Try it, worked on all of mine.

rmeldo, there's a "Pound Shop" in Boundry Road.
Interestingly though, most items are £2.50 (trades description??).
They often have some end of line bargains.
 
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rmeldo

Senior Member
I asked for the name because if it was called "Poundland" I would have known where it is. If it is called exactly "Pound Shop" Then now I know too thanks to BB.
 
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Andrew Cowan

Senior Member
Dippy: Agreed. To load 4605 in 10 seconds would require a rate of 460Hz. If it takes much longer than that, it affects what it can be used for.

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Peter-C

Member
Success

Thanks to all who responded, particularly BB who pointed me to the working solution.
For the benefit of those who come across this thread in the future I will detail the working system:
The unit is a 1.5V exercise step counter. The reset operates by tripping two connections. The circuit runs from a Picaxe output pin 2 to a 22k resistor and on through a 0.1uF cap (in series) to the positive reset connection in the counter. The negative reset connection goes directly back to the picaxe 0V.
The counter function operates with two connections as well, but only one needs to be hooked up. I ran this from another Pic output pin, through a 4.7k Resistor to a 0.1uF cap and on to the counter terminal. The second terminal in the counter had to be left floating as it upset the LCD firmware. The counter is obviously taking its ground from the other two terminals and works very well. Code as follows
main:
high 2
pause 500 'to clear display
low 2
pause 750 ' 750 ok- a shorter pause causes under reading
readtemp 4,b3
sertxd (#b3) 'to check accuracy of displayed temp.
for b0=1 to b3
high 1
pause 120 'timing is critical to avoid bounce doublecount
low 1
pause 120
next b0
sleep 30 'low power for 70 secs
goto main
I have it displaying a readtemp variable, though it could equally display a series of variables, one at a time. It takes a couple of seconds to increment the display up to the desired number so this method is probably best suited to outputting numbers that are lower than 100, and don't change too quickly.
I hope this proves useful to somebody else looking for a very cheap form of readout.
Off to Poundland tomorrow to see if they have any more!
 
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eclectic

Moderator
Peter.
Great find and thanks for the research.

I've probably missed it, but,
how are you powering the LCD unit?

Battery or potential divider?

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BeanieBots

Moderator
Glad you got it working.
Try a lower capacitor value to avoid double counts. 0.1uF is a bit high.

Also worth checking cereal boxes for 'freebies'.
That's how I got a few for the sake of a couple of tokens + postage.
 
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Peter-C

Member
Eclectic -The LCD unit is powered by its own internal button battery, which claims to last for two years at 10,000 steps a day. It seemed an unnecessary complication to try powering it externally on that basis.
 
hippy

hippy

Ex-Staff (retired)
For the low-current need 1V5 MSR radio time receiver I was playing with it was suggested to use a simple resistor divider. That worked well, 2K2 to +5V, 820R to 0V, ~1V5 from the centre.
 
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