Need to drop voltage

[Tim Vukman]

Can someone recommend the best way to lose about 1/2 a volt or so?

I am trying to use a 74LS32 quad OR gate to combine 4 imputs to 1. The logic is straight forward, but I am presenting 1.08 volts to the inputs of the 74LS32 before I push a button to make the input high.

The 74LS32 will read anything above .8 as a logic high.

Therefore, my outputs from the 32 are always going to be high.

All suggestions welcome, even if circuit changes required.

Thanks

Tim

 

[tsweeney]

Any problems with using a diode? Not sure of the ideal type.

 

[hippy]

Run the inputs through diodes joined together ( and maybe through another 'for good luck' ) and you might not even need an OR gate, but you will need a ~10K pull-down on the input pin.

 

[Tim Vukman]

Thanks!

It looks like the diodes win.

Just so I am clear. I have my inputs coming in to the input pins and then I take a diode from each input pin and tie 1 end together (the end with the bar I presume).

Then I run another diode from the joined end to the final input (bar end to the final input) and I hold the final input low with a 10K resistor.

Please confirm

Thanks

Tim

 

[hippy]

Yes, that's exactly how I'd try it.

 

[Mycroft2152]

Just curious. where are these inputs coming from?

Myc

 

[Tim Vukman]

Thanks Hippy

Hi Mycroft2152

The inputs are coming from a keypad. I am using 4 outputs to send the 4 rows high 1 at a time in a constant loop.

Each of the columns goes to a 10K to ground and also to an input pin on the 28X.

I can pull a unique key number for each of the keys and save it in a variable. My issue is that I only have one interrupt to work with.

That's what lead to the idea on "OR"ing the inputs into one. That's what I will do with the Diodes.

Since any keypress will send the final single input high, I hope to use that to drive the interrupt and then capture the key number in a variable

Thanks for asking

Tim

 

[Tim Vukman]

Hi:

The 5 diodes are in place and each of my 4 inputs pulls the 5th one high! Thanks

Now, if I can figure out how to make the interrupt routine do what I want, a major hurdle will have been crossed!

Tim

 

[Mycroft2152]

Glad to hear it works!

Is it a hex keypad?

If you want to reduce the overhead on the PICAXE, consider the mm74C922. It's a hex keypad interface chip with 4 binary outputs and a keypressed output.

I'm not saying don't do it all in the PICAXE. Just offering an alternative. Sometimes its better to add in one of these simple chips.

Myc

 

[Tim Vukman]

Sorry for the delay

No, it appears to be a binary keypad. I wouldn't mind reducing the pin count. I will look up the chip you mentioned.

Thanks

Tim