Versatile Voltage Divider / Isolator Interface for Arduino / Raspberry Pi

In working on various projects, I often need to interface signals to the input of an Arduino, Raspberry Pi, BeagleBone, or custom digital input. I wanted to be able to handle signals from 5V DC, 24V AC (sprinkler controller valves), up to 240V AC (to monitor my well pump run time). It needed optical isolation and an indicator LED to make it easy to see when a signal is active. I also wanted screw cage connectors so I did not not need to deal with crimping headers and larger wires can be used when monitoring a signal some distance away. So I put together a basic building block type circuit board that can be adopted to a wide range of uses. Below is a image of what it looks like. You can see it uses all through hole components. Since I will be using this in many different ways, I did not want to hassle with tracking down SMD components in the different values I will need. Also through hole makes it easy to change the values in the field when I am away from surface mount soldering tools.


If you just want to see how it works and make one yourself, here is the schematic. The values will vary and not all components are needed depending on the application.

I wrote up some application notes to keep track of the values and part numbers I have used.

Or if this looks useful, I put the blank boards up for sale on Tindie:


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Solar Car Cooling Fan

I live in Southern California and getting in to your car after it has been sitting in the sun is only enjoyable for those that consider hanging out in a sauna while dressed in your finest Brooks suit a good time.

So I did some searching on the InterWeb thinking surely someone is selling a solar powered fan I can stick in my car so it will not be so hot when I get in. I did find a few solar car fans for sale, but they all have horrible reviews. So I thought it must be that they are using cheap fans and solar panels that are too small. I envisioned that if I used a quality fan connected to a large solar panel, it will keep my car cool, and surely others are looking for the same thing–I can sell it and make millions.

So now you know the dream, here is the reality: I took a high quality fan, mated with a large solar panel that covers the entire rear dash of my car, and left a data logger in my car to see how much it lowers the temperature. The result, I think I was able to notice a 2 – 3 degree difference. I say “I think” because day to day the outside temperature is different and you can not be sure if the difference is from the fan, a lower outside temp, or some combination of the two. The fan works, you can feel the hot air flowing out of the car. I guess it is just the laws of physics–the heat energy coming in from the hot sun is dwarfed by the amount of heat the fan can pull through the cracked windows.

You can see the pictures of what I put together below in case you want to give it a try. With a bigger panel and more fans, I am sure at some point you can get a decent reduction in temperature. Actually someone has already done this:

Look at features.

Here is what the Maybach does: “In sufficient lighting conditions, five rows of six silicon solar cells generate approximately 60 watts to power the vehicle ventilation fans, after the engine has been shut off. Solar module is included with the Electrotransparent glass roof package.”

If you are thinking, forget building a solar panel power car cooling fan, I’ll just get me a Maybach. Read all about these rolling luxury transportation systems here. I quickly came to the conclusion buying some extra Brooks suits to soak up the sweat would be a lower cost option.

Car fan running and off on alternate days

As for the millions I was going to make selling Solar Powered Car Fans, I guess I will live under the weight of my broken dreams and get back to work on my other side projects:

Schedule My Employees

High resolution control room amplifiers

Used by some of the world’s top mastering and recording studios

(Pic of my engineering/test unit)


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Zero Vampire Drain SLA Battery Charge Controller

12 volt sealed lead acid batteries are very useful for many applications, but many charge controllers put a small drain on the battery. Once this drain has emptied the battery, it will be ruined if not immediately recharged. But with a charge regulator that does not put a drain on the battery, it can store for months and still hold a decent charge.

So I put together a simple shunt regulator to test out the concept. So far it has been working well. I can leave it in the garage for months at a time, and grab it for camping trips, etc.

You will need a heat sink big enough to dissipate the excess energy from the solar panel. The piece of sheet aluminum seems to be enough for the 5W solar panel I am using to charge a 12V 7AH SLA battery.

There are pros and cons to this design. Pros: simple, no vampire drain, inexpensive. Cons: it does not cut the battery off when it runs low, you need to watch that, you will need a big heat sink if you have a big solar panel.

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SUN-2500P Power Inverter Remote Control

In searching for inverters that could run a 3000 – 4000 watt load in an off-grid solar system, I found some low cost inverters on eBay (SUN-2500P is the model I see on these). An issue soon came up after putting them in service. If anything happens, and the inverter trips, it never comes back on-line until I go out and manually reset the inverter. This is a showstopper for a number of reasons, so I set out to find a way to auto restart the inverters.

The units are controlled by a DPST switch. I wanted to have the units normally running, or normally off depending on the state of a toggle switch, and have a signal put the inverters in to the other state to to either reset or power on the inverter.

So I took a DPDT toggle switch and a DPDT relay and connected the throws together. I spliced the poles in series with the switch already in the inverter so if it is turned off, the inverter goes off regardless of the relay status. Made a small assembly and mounted it by removing one of the air slot cross members.

Here is an idea of what it looks like:

I connected wires to the really coil, put a strain relief zip tie to the assembly and ran them out of an air slot. While this is not the most professinal modification, it saved me from a show stopper situation with these inverters. I am testing these running 2 units ganged together to run a 3KW load with some success. The long term reliability of these inverters is still in question so in the end I my end up with a higher end solution. Time will tell.

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Repair Yihua PS-3010D Power Supply Stuck at 50 Volts

I have a Yihua PS-3010D 30V 10A power supply and one day it would only output 50V. After doing some measuring with my volt meter it looked like the top supply rail was shorted to the output. I could see a bunch of power transistors on a heat sink and assumed they were the output devices. I removed 4 screws at the base, 2 at the top that the control board was mounted to, cut a few zip ties, and used some tin snips to make the hole at the bottom the wires were running through into a slot. Then I was able to carefully work the output stage out enough to work on it.

I could see that the transistors were being used as emitter-followers with the emitter of one of them being used to drive the bases of the other 3 wired in parallel with sharing resistors on the output.

I removed the orange wire you see running around the bases and did a back to back diode test on each of the 3 transistors wired in parallel. I could see one was shorted and the others looked OK. I went to my junk box, found a 2N3055 transistor (with a date code from the 1970s!), put it in, soldered the orange wire back into place, and…the power supply was back on-line. My best guess as to what happened is I connected the power supply to a circuit that had a charge on it and it back feed the power supply. So took a TO-220 diode (FERD40H100STS) and connected it so it will dump back fed power to the big filter caps.

Not sure if this will work or not, and I am not going to try and blow up my power supply to test it. But the diode has not caused any problems so I will leave it in and hope it saves me next time.

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Build a Cheap 24V 20A Battery Charger

I have a 24 volt battery bank I use to run an off grid well pump. On cloudy days, sometimes I want to run the well to test out sprinklers, etc. So I wanted a battery charger that could put out 20 amps if I connect it to my generator. Ends up you do not go to Wally World and get a cheap 24V battery charger. The ones I found were expensive, and put out less then 20 amps. So I bought a used 24 volt transformer on eBay for $50, got a 40 amp diode, cut up some wood I had in the corner and put it all together to see if it would get the  job done.

The diode PN I used is FERD40H100STS. When I hooked it up, it worked well. With the well pump running and the battery bank voltage dropping to about 22V, this charger was pumping about 450 watts into the battery bank. Close to the .5 KVA rating for the transformer. As the batteries charged, the power went down. Once the batteries were mostly charged, it was putting out about 200 watts. The plate the diode is mounted to is the positive out, the other wire coming out of the transformer is the negative connection. As you can see, I have 1/4 ” bolts for the + and – so I can put jumper cables between the charger and the battery bank.

If I had to build it over, I would have put a fuse somewhere (you should), and would have made the plate the diode is on bigger (it gets hot).

I have some extra diodes. If you PayPal $4 I will drop one in the mail in the USA. Or you can find them online. The main thing is to watch eBay or such sites and find a good deal on a used transformer. New they are pricy.


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Cheap Trick to Remove a Frozen Ferrule

Replacing a leaking cutoff valve for a toilet or faucet is pretty simple, expect for when the old ferrule/compression ring get stuck on the water pipe. This happened to me the other day, and I did not have a puller tool available, but an idea popped into my head that I might be able to use the old valve to pull off the frozen ferrule. I gave my idea a try, and it worked. Here is what I did:

It ends up 1/4″ washers fit perfectly inside the old valve. I put enough in so that the end of the pipe will push against washer and the nut will pull on the stuck ferrule.

Tighten the nut until the ferrule hits the valve, take the valve back off, add another washer, and repeat until the compression ring is almost off the end of the pipe.

Now a gentle twist and pull should take the ferrule off. Clean up the pipe with some sandpaper, and you should be ready to put on the new valve. Not a bad trick for a DIY plumber to know about.

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Running a Digi TransPort WR11 XT on USB, 12V and 24V power

Update 2

Digi has a new product to address this need.
Digi PN: 76000976
I have it running in the field for about a month with any issue.



This setup worked great while running in my workshop, but once put into service in the countryside, the WR11 went dead after about a day. My best guess is the cell signal is weaker/different in the countryside and the WR11 might really pull some big power at times, more then the USB charger can supply. Now that the days are longer and the rain has gone, I have moved the WR11 back to AC power. I will continue to work on this project to get ready for next winter when I need to use power judiciously.


I have a remote hobby ranch that I monitor over the cell network. I have found the Digi TransPort WR11 XT to be a solid way to get a connection to the cell network for sending logging data back to a central server. Long story, but in the end running the WR11 on AC power was not good situation as the primary power is a 24V solar system. It ends up the most difficult part of the project was tracking down the special locking barrel connector Digi uses. After ordering a handful of connectors, paying shipping, and waiting after each to arrive and not fit, I finally found the right one:

Mouser PN: 806-KLDX-PA0202-B-LT

Below is a cable I made with the locking barrel connector.
Specs say the WR11 can use up to 15W of power, so to run on my 24V solar system, I used a 2.4A 12V – 24V cigarette lighter style auto USB charger. I run on one SIM card and find the power usage stays under 3W. Time will tell if I will need to find a beefier USB charger, but so far so good. So if you find yourself trying to run a WR1 XT on DC power, the connector PN above may save you the time, hassle and money I spent.

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Script to Limit Number of Files in a Directory

I have some jobs that run and will fill up disk drives if not pruned.

I put together the simple script below that I can call with a directory and the number of files I want to keep. The oldest files will be deleted. I add it to cron jobs with a command in this pattern:

~/ ~/public_html/FrontDoor/Camera\ 01 200
Note: there are 2 params, the \ escapes the space before the 01
Copy into


for line in `ls -t "$1"`
  if [[ $Cnt -gt $limit ]]
    rm "$dir/$line"
    echo "rm $dir/$line"
  Cnt=`expr $Cnt + 1`

While I know this is not the most professional way to do this, I wanted something very simple I could play with and should have few dependencies so I could use it on shared servers that I do not have control over.

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Lessons Running IP Security Cameras Off Grid

On a small off grid hobby ranch, running IP cameras to keep an eye on things proved a bit trickier then planned. Durning the summer the solar panels produce enough power to run a well, a data logger, a camera and a cell modem to send data once an hour.

In the winter however, the shorter days, lower sun angle and rainy, cloudy days dramatically reduce the power produced. The well system is designed to only run when enough power is available, but the data logger and camera run 24/7 and can run down the batteries and cause a system shutdown after a string of cloudy days.

In looking over where power was being consumed, an IP camera running on PoE (Power over Ethernet) from a PoE switch was consuming 10 watts 24/7 and pushing the system over the edge. So I set out to measure the different ways the camera could be powered and see if the consumption could be reduced. Below are my measurements:

Camera: HikVision DS-2CD2032F-I, Switch: TP-Link TL-SF1008P
PoE switch only 2Watts
Switch & 1 camera day (IR LEDs off) 6W
Switch & 2 cameras day 9W
Switch & 2 cameras 1 day, 1 night (IR LEDs on) 11W
Switch & 2 cameras night 13W
1 camera running on 12V power plug day 2W
1 camera running on 12V power plug night 3.3W
Non-PoE Switch TRENDnet TEG-S80g running on 5V DC (not on 120VAC wall wart)
No ports on .85W
1 port active 100Mbps .9W
2 ports active 1-100Mbps, 1-1Gbps 1.2W
3 ports active 2-100Mbps, 1-1Gbps 1.3W

So it looks like like the lowest power operation can be found running without PoE or AC but instead using the DC power plug, staying in day mode if possible and finding the lowest power non-PoE switch you can find. When running on-grid, these type of power differences do not amount to much. But when running on a small off-grid solar system, every watt counts.

Here is a site with lots of info on deep cycle batteries I found useful:

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