I finally got the Velleman FM radio kit working satisfactorily this week. My initial problems had been excessive noise and lack of sufficient time to debug. The radio specs show an input sensitivity of 5-10mV. They recommend (in broken english) supplying a signal with double the impedance of the signal generator you’re using (i.e. the mic or amplifier). Since I want to use a microprocessor to generate the sound, this presented problems.

The BX24 has no info on its internal impedence, and it outputs at a whopping 5 volts. By the time you bring this down to 5mV, you’re current is at virtually nothing. I hooked the BX24 up to the radio input through a bunch of resistors to bring it down to about 1 volt, and then tried it straight in (yes, supplying 1000 times the recommened voltage), and the radio didn’t seem to care which way it was plugged in, so I’m sticking with the straight input.

The kit also came with a diagram of how to wire up an electret microphone, and this works very well, although it’s not what I’m interested in right now. The circuit in the kit also includes a pot to vary the input level, and this seems a bit buggy and erratic, but does help attenuate the signal somewhat.

As a tip to anyone using a radio kit, don’t use the same power supply for the radio transmitter and the microprocessor and the radio receiver. Any combination of these two will lead to confusion and a bad signal. Each one must have its own power supply.

In other news, in our Spatial Design class last week, I designed a piece that uses joints and mechanical motions to kick a leg repetitively. The piece was a kind of illustration of kinematics of the leg with each joint following the path of a groove cut into foam. This was referrential to post-WWII artists who fetishized dismembered body parts, as well as a memory of various dissections (including frog legs and assorted brains) I was subjected to as an undergrad. I hope to do more work involving mechanical motions. As a tip to anyone interested in mechanical movements: the book “507 Mechanical Movements” is an amazing collection of the most common mechanisms you’ll find inside just about everything.