How to connect higher voltage to PIC chip input reliably ? Answered
The PICAXE 08M2+ IC is just an 8-pin flash PIC with nanowatt...
I have been working on my 3000mW handheld laser monitoring system for ages. It is a picaxe 08M2 connected to an LED, a buzzer through a transistor, a thermistor, and an 8.4V input.
It is almost perfect, the problem I have isn't really serious, but I want to make it perfect.
Now read carefully what happens:
The picaxe comes on when the laser's "armed" keyswitch is turned. An on/off pushbutton then controls turning the laser on/off, all the while the picaxe is still on.
The PICAXE runs off a 78L05 connected to 8.4V. If I turn on the PICAXE, it works all fine as usual. When I turn on the laser (1.5A load), it is still perfectly fine due to the 0.22uF capacitor I put on the PICAXE 5V rail!
BUT, if the laser pushbutton had been left on, and you turn the armed switch on, the laser comes on as usual, but the PICAXE doesn't boot. If I turn the laser pushbutton off then on, even for a millisecond, the PICAXE boots and works fine from then on.
Now here is how it is wired up, I want the PICAXE to know when the laser is on, so I connected the pushbutton to INPUT3 on the PICAXE so INPUT3 gets power along with the laser when the pushbutton is pressed.
I realise that the 8.4V input is too high for the PICAXE, so I experimented with resistors and discovered that a 47K resistor between the 8.4V and the INPUT3 worked fine (with a 10K pulldown).
I have checked and it is not a software problem.
What I don't get is why the PICAXE stays working fine when the laser load is turned on, but not if they are both turned on at the same time.
The attached drawing is to help explain my set up.
See how the PICAXE only fails to boot if the laser on/off switch was already on when the main keyswitch is turned on. If I turn the laser pushbutton off then on, even for a millisecond, the PICAXE boots and works fine from then on.
I hope it wasn't too hard to understand...