K55 Radar RS-232 "Upgrade"


friend of mine wanted to connect his laptop to a K55 police radar set in order to convert it into a data logger to document people speeding through his neighborhood. As a person who also gets very upset about this bad behavior (even by my own neighbors), I took special interest in the success of this project. I developed an RS-232 interface for this dated, but still quite usable, equipment. I also wrote a piece of Windows software that takes a still image from a USB camera and stores it to hard disk with the time/date and speed superimposed. A .CSV file for spreadsheet input of time/date/speed is also created. The "violation speed" is user-settable, making this easily-hidden assembly nearly the technical (but not the legal) equivalent of those hated robot radars!

Figure 1. Early K55 Police Radar

Figure 2. Board and Wiring in Unit



This particular K55 model is a very early (ca. 1971-74) example, and used discrete counter logic. Worse, it used early 10 volt 4000-series CMOS circuitry. There was no schematic available, so a small amount of reverse-engineering was in order. I used 4010 buffer/level shifters, inputting BCD numeric codes from the 4511 chips driving the ones and tens digits, along with their latches. This now TTL/5V CMOS-compatible output was input to an Atmel "AVR" type 2313 microcontroller operating at 3.58 Mhz, which interprets the latched BCD information and sends ASCII output in the form of XX<cr> at 2400,n,8,1 to a MAX232 type level shifter. Blanking is represented by a "-1". Data is sent upon any display change. The RS-232 signal, with no handshaking, is brought out to a barrel plug and a cable with appropriate "cheater" wiring so that it will work with or without handshaking on the receiving end. It is up to the receiving device to buffer the output.

I usually use Microchip/Ubicom microcontrollers in projects of this type and program them in C, but I got in some Atmels at a good price and there are very good (and low/no cost) BASIC (seriously, BASIC) compilers available for them. So, in an unusual turn of events, this firmare was written in a BASIC which emits assembler! The quality of the compiler was fairly poor, both in capability and quality of output, but it does work, and in its defense, it was free.

A small 78L05 TO-92 package was sufficient to power the +5 logic. I don't think the board puts more than 20mA additional load on the 10 volt power supply.

The K55 was built with multi-PCB module technology mounted on a frame with interconnects. Space was very tight, so I made a PCB for the adapter instead of wire-wrapping a board. The board was mounted via standoff screws on an existing board. Connections are via homebrew mini-connectors made from gas-tight IC sockets.

Of note is that this was my first double-sided board made at home with a laser printer; the tricky part was registering the sides correctly, but the quality of the finished board was not bad even considering numerous narrow traces and my use of a new transfer paper. I did, of course, have to solder both sides of many pins due to the lack of through-hole plating.