Grain door status monitor

[wllshaw]

Gentlemen:
It's has been a while, but in the mean time I have learned a bunch. I am at a loss right now though, and I have a question. I am working with a 18x Picaxe. I have successfully down loaded programs into it and I currently have it controlling transistors that turn 12v lights on and off. I am now trying to use the 1702 low drop out voltage regulator to power the picaxe off of a 12v battery. I have the data sheet in front of me and I have successfully hooked the regulator up. Using a multimeter I am getting 5 volts out of the voltage out line. I have the 5 volt line connected to 5 volt positive rail that the pic chip is connected to. The ground for the 12 v battery is connected to the negative rail that the pic chip also connects to. When I try and down load a program into this it doesn't work. I have checked the voltage on the rails going into the pic and I am getting 5 volts. Now for my question. It won't work as I have described it, but if I connect a 4.5 v battery to the same rails that the 12 v battery is connected to, the program down loads great. Before you ask, I don't have a schematic to down load. If you would suggest a program where I can draw one up, I would be happy to make one up and show you. When the two batterys are connected to the rails the voltage is the same. The only difference is that it won't down load unless the 3 double A batterys are connected. There is no voltage difference on the rails. Any help would be appreciated.

 

[BeanieBots]

Have you got all the other (capacitors) components shown in the regulator's datasheet?
What is the regulator's part number. I'm sure sombody will look it up and tell you what caps need to be fitted. A bog standard 7805 tends to oscillate if it does not have 100nF very close to its output.

Could also be an issue with the RS232 volatge comming from your PC. If it's on the low side (~5v) it can cause download errors when Vcc is 5v.
IF and ONLY IF, your PC is putting out 5v RS232, then try dropping the 22k to 10k.

 

[BCJKiwi]

If that's an MCP1702 you may be living dangerously as it has a maximum input of 13.2V and an absolute maximum of 14.5V and a vehicle will produce more than that - suggest you change it for an LM2940 which is designed for automotive use.

 

[wllshaw]

Thanks for the advice. I am using the 1702 so I will replace it with your suggested part number. Right now I am using double A batterys in series for the 12 volt supply. This evening I did manage to get it to work. I added a resister to the voltage out line and dropped the output voltage to 4.9 volts and it worked great. I will have to reread the pic maximum input. I thought it was 5.5 volts, but obviously I was mistaken. Thanks for your input. At this point I did not have the capacitors added because I was working with a known supply, but I was planning to add them before I hooked it up to the tractor voltage. Once again, many thanks

Will

 

[BCJKiwi]

Theoretical max is 5.5 but <=5.0 is recommended max - why stress out the device and soak up more current for no benefit?

When the Reg is changed, ensure you follow the Capacitor recommendations for that specific regulator from it's data sheet. The LM2940 likes tantalums and is different from other regulators.

 

[boriz]

@Dippy
“Hey, how about a POV 'scope.”

Man. I was half way to building one when I came across a cheap ‘real’ oscilloscope. It was gonna be a rotating stick with a line of 3mm LEDs on it. All back when I started electronics as a hobby.

More recently I built a LASER POV ‘scope using a cheap LASER pointer, a small mirror mounted at the hub of an old PC fan and a heavily modified speaker. I removed the speaker’s cone, leaving the coil suspended by a flexible ring. Then fixed a small balsa lever with one end loosely fixed to the speaker frame, the other to the coil, so that the lever would tilt slightly according to the coil current. Glued a tiny mirror onto the lever. It worked ok. Put on a bit of a light show with a music-wave cast around the walls of the room. But I could only get it to work for low frequencies, maybe <500Hz.

 

[wllshaw]

BCJ:
Thanks for the info. I reread the section on voltage regulators in manual one last night. I realized I was right about the 5.5 volts being a max, but concluded, as you had said, that less then 5 was better. What I hadn't realized was that the capacitors helped smooth the supply of the regulator. I thought their only job was to protect the regulator from voltage spikes coming at it from the main supply. Once again, many thanks.
Will

 

[BCJKiwi]

In the case of three terminal regulators, the capacitors also may do other things - each regulator type is different.

In general, the one on the input smooths out the supply going into the regulator and the one going out copes with variations in the load.

I see many circuits where the output side cap is really beefed up but often this is not necessary.

e.g. Have just installed a 7809 to supply a couple of cooling fans with no capacitors at all - why not - well the data sheet says I don't need them as the supply is a 7824 and is real close, and the load is steady and light.

However many regulators require output caps of a specific size and type to make them work and not oscillate - the LM2940 is such a regulator. It also has specific requirements - it must be => 22uF and low ESR - hence the tantalum. there are other options but this is the simple answer.

so the answer is (no not 42 in this case) read the data sheet and do what it says.

 

[BeanieBots]

Bottom line, a regulator doesn't make a power supply any more than a transistor makes an amplifier. It is just one little component within the design.