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Post by PaulHansen » Sun Sep 21, 2003 4:07 pm

Hi<p>I am fairly new to electronics and my question is this.<p>I have a treadmill at home that came with a small
wrap around your chest gadget that transmits your heart rate to a watch you wear on your wrist.<p>As the heart rate goes up while running so to does the read out on the wrist watch.<p>My question is what kind of technology is sending the info from the gadget around your chest to the one in the watch. What kind of radio wave is this and where could I learn more.??<p>Cheers in advance everyone<p>Paul Hansen
New Zealand

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Re: How

Post by haklesup » Mon Sep 22, 2003 6:02 pm

The chest strap has two rubber pads that are actually electrically conducting electrodes. You need this material to be moist to make good contact to your skin. <p>In any case, once you press the chest piece to your skin, a small current (you cannot feel it) is put through your skin from one electrode to the other, this turns on the transmitter. Each time your heart beats it creates an electrical disturbance which is sensed as a signal on the electrodes. This signal (mostly just a square wave) is transmitted to the wristwatch on an RF carrier (I don't know the frequency but it would be very low power). The counter and clock in the watch get together to compute and display the pulserate.<p>wet your thumbs and press one to an electride and tap the other electrode with the other thumb. The heart symbol should flash for each tap. You are simulating the heartbeat this way by making and breaking the continuity of the loop.<p>I will guess the carrier is FM but it could be just about anything they wanted since the signal is just on/off. Probably same type of signaling used in toy RC cars or RF remote controls. No modulation is really needed if they just turn the carrier on and off. <p>Anyone else have insight on the RF link?<p>For other applications, it could count the frequency of any event in the 60 to 200hz range(approximate range of heart rates)

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