The compact circuit presented here is perfect for receiving the signals from pulsed fixed-frequency transmitters. Chest straps from several well-known brands (Polar, Huger, Kettler, Crane, Outbreaker, …) transmit a short signal burst with a frequency of 5.3 kHz. These signals can be received and used in your own projects, as the author shows on his website [1].
The circuit uses a ferrite rod with 1000 turns of 0.2 mm enameled copper wire and a (tuning) capacitor to receive the signals. The value of the capacitor (22 nF) has been selected for use at a frequency of about 5.3 kHz, but this can, of course, be adapted for use at different frequencies. The received signals are amplified by the opamp (IC1), after which a NAND gate (IC2) turns them into a nice waveform with straight edges. For the supply you can use any DC voltage source in the range of 9 to 18 V. There is a board layout available [2], which can be ordered via ThePCBShop [3].
Web links
[1] http://www.elektor. com/080093 [2] www.thepcbshop.comThe NAND gate or “NotAND” gate is the combination of two basic logic gates, the AND gate and the NOT gate connected in series. The NAND gate and NOR gate can be called the universal gates since the combination of these gates can be used to accomplish any of the basic operations. The Logic NAND Gate function is sometimes known as the Sheffer Stroke Function and is denoted by a vertical bar or upwards arrow operator, for example, A NAND B = A|B or A↑B.
