Problems with 2.3" 7-segment LCD, 4026B and PICAXE14M2

[jproehl]

I am trying to make a large display of distance data from an ultrasonic detector connected to a PICAXE14m2. I'm having trouble figuring out how to connect the PICAXE14M2 running at 5v to two 4026B counter microprocessors and 2.3" 7-segment displays running at 7 volts (the large display uses 4 Red LEDs in series for each bar in the display, so it needs 7v). What I have done is put the 4026B and LCD display pairs on their own board and power them with 7v (the 4026B stats sheet I read says it can handle up to 20v). I have the PICAXE14M2 on an AXE117 wired to 5v supply and I have the clock and reset lines on ports B.1 and B.2 respectively. These two feed to the LCD board and connect into the clockin and reset lines on the first 4026B, as described in Chapter 3 of the PICAXE manual.

What I find is that after all is wired up, I get both 7-segment displays showing zero on power up. When I run the code on the PICAXE14M2, I only see intermittent weak flashes on the center horizontal bar (the only one not lit when the LCD is displaying "0"). I suspect that the 5v clock and reset signals may not be adequate to cause the 4026B's to operate the large LCD displays. Does this sound reasonable? If so, would boosting the clock and reset signals to 7v solve the problem? Further, if this is the solution, how would I best accomplish this on the AXE117 board?

I've attached a file showing how the LCD board is wired. At the top center of the diagram, you'll see a series of 7 pins, the top 5 of which take the external feeds from the AXE117 to control the signals. The bottom 3 of these 5 pins are meant the drive a couple of 3 led light boards (red, yellow and green), and the top two pins are the clock and reset inputs also coming from the AXE117 The bottom two pins of this 7 pin header are the 7v power lines.

Any ideas? Thanks!

 

[hippy]

From one 4026B datasheet I looked at, the minimum voltage for a high input has to be 3.5V when the 4026B is operated at 5V, 7V when operated at 10V, and 11V when operated at 15V.

When the 4026B is operated at 7V it would seem that the voltage in to be seen as high would have to be pretty close to 5V so it could be it just doesn't have the drive voltage.

You could bump the voltage up using a transistor, voltage level translator chip, but it may be possible simply by using the PICAXE pin set only as INPUT ( for high output ) and LOW with a 22K pull-up to 7V.

 

[jproehl]

From one 4026B datasheet I looked at, the minimum voltage for a high input has to be 3.5V when the 4026B is operated at 5V, 7V when operated at 10V, and 11V when operated at 15V.

When the 4026B is operated at 7V it would seem that the voltage in to be seen as high would have to be pretty close to 5V so it could be it just doesn't have the drive voltage.

You could bump the voltage up using a transistor, voltage level translator chip, but it may be possible simply by using the PICAXE pin set only as INPUT ( for high output ) and LOW with a 22K pull-up to 7V.

Thanks, Hippy. I'll look into doing this. Just for my education, isn't the Darlington array on the AXE117 just an array of transistor pairs? I'm wondering if there is a way to use the 5 volt PICAXE signal pins to trigger the Darlington output pins to provide a 7v signal to the 4026's? If so, how would I wire up the AXE117 to do this (I'm still a newbie on this)? Thanks!

 

[hippy]

I had misread your AXE117 as AXE171!

With the Darlingtons of the AXE117, and if you have connected to those, they are operating as pull-down only so that might be the problem. Add 10K pull-ups to 5V for both lines and that might fix things. Otherwise connect the 10K to 7V and try again.

 

[jproehl]

I had misread your AXE117 as AXE171!

With the Darlingtons of the AXE117, and if you have connected to those, they are operating as pull-down only so that might be the problem. Add 10K pull-ups to 5V for both lines and that might fix things. Otherwise connect the 10K to 7V and try again.

Thanks, but I'm not entirely sure how I connect the pull-up resistors to the circuit (Newbie!). Could I connect the output pins directly on the PICAXE defined as clock and reset to the first 4026B? When those pins go high with the pulsout command don't they get to 5v or do they require a pull up to 5v? I assume I cannot power the PICAXE with 3-AA batteries as that only gives 4.5V, right?

I'm also unsure of your last statement: If I connect a 10K resistor to 7V where does the other end connect to?

BTW: I determined that I can light the 7-segment LED display with a 6v wal wart, so I'm now doing that, which I assume should drop the activation voltage of the clock and reset signals below 5v.

 

[westaust55]

Looking quickly at this topic:

A schematic is always better and easier to follow than a PCB layout for fault finding - or post both.

FOr the CD4026B chips you have pin3 tied to Vcc so the display outputs will always be active.

However, if you are going through Darling Transistors on the AXE117 then not only are these outputs open collector (so the outputs are floating when the input is low and pulled low when the input is high), they are in effect also inverting the signals.

Adding a 10 k resistor between 5V and the used Darlington outputs will overcome the floating output aspect. Then when the input is low, the Darlington output will be pulled high byt he resistor.
But you still need to consider that the signals through the Darlington are in effect inverted.

The better option would be to connect to the holes between the PICAXE socket and the Darlignton Transistor array socket.
Another option to protect the PICAXE IO pins is to remove the Darlington Array and use some 330 Ohm resistors across the socket
See:http://www.picaxe.com/docs/axe117.pdf