Wednesday, November 2, 2016

Let's build a field strength meter

A short while ago I watched a video that was floating around Facebook that detailed how to convert a cheap (freebee with coupon) Harbor Freight multi-meter into a field strength meter. Seeing as how I plan to get back into antenna building, and that I literally had ALL of the parts needed, I thought I would give it a go.

 My schematic is a bit different than the one outlined in the video. There is one capacitor that is supposed to be in series with the antenna. I simply forgot to install that, so I just removed it from the schematic altogether. Below is a picture of my crappie schematic.

I think the most complicated part of this was trying to get everything mounted and fitted into the box. The holes drilled for the antenna and switch seemed ugly to the eye but work. I had difficulty getting the switch to fit properly, so I made the hole larger with a pair of wire clippers.

 The whole circuit is wired into the meter inputs via a switch. The switch gives you the ability to turn the Signal Strength measurement ability on or off, so it can still act as a multi-meter.

  Here we can see the meter all put together and taking a reading of a small FRS radio transmission. I hope to use this in my efforts to map out antenna patterns in the future. This is a good one night project if you have all of the parts laying around.

Friday, October 28, 2016

Some Radio Recognition

Well its been quite some time since I posted last. I'd really like to post more but as it stands, time like all other things is a commodity in my life. I do currently have a few projects going on and I would like to get them up on here. Perhaps around Thanksgiving I will have more time.

Anyway, I noticed my account on QRZ.COM had a link to this page and figured that if I have anything radio related I might want to post that here since folks in the amateur radio community might happen upon it.

Here are two little videos I made containing the recording of my submissions to BBC World Update Soundscapes.


My reception of the US Navy submarine transmitter in Cutler, Maine

My reception of the ISS when it was sending Slow Scan TV Transmissions

Friday, May 8, 2015

Roomba Hack Part III

Well it’s been more than a year since I have touched the Roomba, and to tell the truth I would have never touched it at all if I hadn’t been cleaning out the garage to find parts for another project.  The last thing I remembered about this is that I had fixed all the mechanical problems it had and even upgraded the firmware to allow me to send commands and read sensor data from it. It’s a shame that I had gone through all of this trouble and it just sat in my garage for over a year collecting dust. While in the process of cleaning I had spared the Roomba the fate of my trashcan. I just couldn’t bear to throw it away. I put too much effort into fixing it. Instead, I merely just set it aside and thought that I would charge up the batteries and at least get it to clean my floor until I could find some way of violating its command structure for my own twisted purposes. About a week after this I finally had an opportunity to hook things up. 

One of the things I love about the season changing from bitter cold to moderately cool is the chance to fire up the grill and eat outside for a change. We had just purchased a new Infra-red grill that I was dying to cook on. I fired the grill up and allowed it to reach a decent temperature. While waiting I went into the garage and attempted to hook the charger up to the Roomba. For some reason the charging light did not come on. I read the voltage from the charger and verified that the charger was indeed working. I then popped the Roomba battery out and read the voltage on it. Zero volts. Great, I just purchased this battery a little over a year ago. I flipped the battery over and noticed that the label read “Made in China”. Well ok that explains it then. So now I needed a new battery, but I wanted to be sure that the Roomba would power up before I dropped twenty-six dollars on a replacement battery. 

I figured that since the battery was completely dead, it wouldn’t be of great loss if I opened up the battery case and attempted to wire together a solution that I could hook up to a power supply for a temporary test until a new battery arrived.
There is really no great mystery here. The battery had three contacts, which I thought was a bit odd. Two of the contacts were clearly for power, 14.4 volts I believe. They were even labeled “+” and “-“. The third contact was on the side and was not labeled. Once I opened the battery case I noticed that this was a little temperature probe and was used to monitor how hot the battery was. I removed the batteries and kept everything else wired as is. I then drilled a hole in an opportune location of the battery case and ran a set of power wires out of it. This would allow me to supply voltage to the Roomba from a remote power supply. I dialed up around 15 volts on my power supply, hooked everything up and pressed the power button. The power indicators lit up and I was in business. Now I can work unimpeded on interfacing the Roomba.
Part of the allure of hacking the Roomba is that the company that makes them iRobot had made this so very easy. In fact they even published a guide on Roomba SCI (Serial Command Interface) communication. In it is a ton of useful information on getting information to and from the Mini-DIN connector on the Roomba. Two things immediately stood out to me that would help me determine what I could use to send commands to the Roomba. First the signal levels had to be TTL (5 Volt) and second the baud rate had to be either 57.6K or 19.2K.
I had briefly thought about using my old Commodore Vic-20, since it supported TTL level serial in and out, and I have had some experience with interfacing it in the past. But I would need some way of powering it, and to even get the 6522 VIA chip up to 2400 baud would require some bit banging. I simply was not up to that challenge. Instead I needed something that was relatively portable, could communicate at one of the two stated baud rates, and had an on board power supply.

OF COURSE, it was so obvious! Why didn’t I think about this before? I could use my Tandy 102 portable computer. It had everything I needed. It could communicate at 19.2K baud, it had a DB25 serial port on the back, it was small and had its own power supply. As a bonus it even had Microsoft BASIC on it so I could easily program it. The one limitation it had was that the Tandy’s serial port used RS-232 level signals for its serial communication. I could remedy this by using a converter such as a MAX232 IC.


The circuit for this is pretty simple, first I wanted to regulate the voltage coming from the Roomba and step it down to 5 volts. From there it was just a series of filter capacitors and a few capacitors to act as charge pumps to get the signal levels up to RS-232 standards.

My first test was simply to hook the Tandy 102 up to the circuit and then communicate with a USB to serial TTL  FTDI device. As you can see everything seemed to be working.

So I went ahead and wired everything up. Now I just needed a program on the Tandy 102 to send commands to the Roomba.

After a little research on the web and reading through the Roomba SCI document I managed to piece together the following BASIC program to exercise some of the more basic functions of the Roomba. Yeah I know its a cheesy program but I really just wanted to get the thing moving.
10 CLS
30 GOSUB 5000
50 IF N$ = "Y" THEN GOSUB 1000
70 IF N$ = "Y" THEN GOSUB 2000
90 IF N$ = "Y" THEN GOSUB 3000
110 IF N$ = "Y" THEN GOSUB 4000
900 END
1010 PRINT #1, CHR$(137);CHR$(0);CHR$(200);CHR$(128);CHR$(0);
1030 FOR I = 1 TO 50 : NEXT I
1040 GOSUB 6000
2010 PRINT #1, CHR$(137);CHR$(255);CHR$(56);CHR$(128);CHR$(0);
2030 FOR I = 1 TO 50 : NEXT I
2040 GOSUB 6000
3010 PRINT #1, CHR$(137);CHR$(0);CHR$(200);CHR$(0);CHR$(1);
3030 FOR I = 1 TO 50 : NEXT I
3040 GOSUB 6000
4010 PRINT #1, CHR$(137);CHR$(0);CHR$(200);CHR$(255);CHR$(255);
4030 FOR I = 1 TO 50 : NEXT I
4040 GOSUB 6000
5010 PRINT #1,CHR$(128);CHR$(130);
6010 PRINT #1, CHR$(137);CHR$(0);CHR$(0);CHR$(0);CHR$(0);

Once everything was programmed and pieced together I was anxious for a test. My battery had since come in the mail and all the planets were now aligning.

There is something a little magical I feel in making an old piece of technology talk to a new piece. Perhaps I just want a little piece of the kid in me to be embellished with the fascination I once had with old computers when they were new. This of course is easy to ignite if I hid it behind some ridiculous project, as I have here. I think this will be the extent of my Roomba hack. I suppose at some point in the future I could get this thing under Bluetooth control, but my fascination with the Roomba is coming to an end. I really need to get back to my Speech Synthesizer project and bring that to a close. Below is a small video I put together of everything working. Enjoy!