534
Item nr.

Gerard Pina Colada Pocket radio

Low cost earpiece radio.

The Design

This Object

As mentioned, the design was inspired by a Kids project. (If you follow the schema given there, please note that R2 must be connected at the lower, not upper side of R1.) It has the standard MK484 tuner design on the left, and a ferrite rod with two coils and L/M switch was used. Usually, the output of such a tuner is very weak, but here it is amplified by the BC337 transistor. The DC current (fixed 0.3mA plus signal dependent DC) from the MK484 runs through R1 to supply the AGC voltage, but the AF current runs through C3 to the base (and the RF current is shorted to earth directly through C1 as usual). The transistor is biassed through R3, which runs about 10µA; the amplification of BC337 being around 100, this makes the drain through the transistor about 1mA. Indeed, total consumption is about 1.4mA from the AA battery. These simple designs are usually switched by pulling the headset plug, but I added a switch in the design.

The values of the components aren't very critical. The AGC buffer C2 is 10nF but values up to 100n can be used. R2 and R3 are 120k but values from 80 to 150k are fine. The DC load R1 is 820R, and 600R to 1k5 will work here. C1, C3, and C4 are 100nF and I don't know how well other values work. I didn't use a board, but free wired the components in a plastic box (from cocobread).

The simple and cheap design works stunningly well! During day time I hear about 6 Medium and 3 Long Wave stations understandibly. In the evening, dozens of stations are heard. On the first evening, I heard England, France, Spain, Italy, Hungary, Holland, Czechia, and Ukrain. It is even possible to connect a speaker and listen audibly in a quiet room. I estimated the output power to be about 2 to 4µW.