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Published on: September 5, 2013 / Comments: 2

Build your own wireless modem using a 555 timer and 565 PLL

Everyone loves the good ol' 555 timer

This build uses good old FSK (frequency shift keyring) technology sending 1′s and 0′s through the air.  Although its applied using infra-red, the same concept is used for RF.  Also Infra-red is still technically wireless (lol).  So on to the build.  Just in-case you want to brush up on your FSK I recommend just straight up hitting wikipedia.   (If not, here’s the TLDR version;  A steady pulse is used to represent a 1, and a variation in the pulse width is used to represent a 0)

On the transmitter side a 555 timer is set up as a Astable Multivibrator, the digital input (fed from say a microcontroller) pulls down the transistor which modifies the frequency of the 555 timer, causing a 0.  Leaving the pin high (representing a 1)  leaves the 555 timer alone.  The data is transmitted via infra-red led.

On the receiver side, a 565 PLL (Phase Lock Loop) IC locks on to the frequency of the 555 timer and outputs a 1 or a 0 when the signal alternates between the 2 frequencies.

This is a great project but mostly as an educational one.  Full post up on gadgetronicx.

2 Comments.
  1. Jasper Tucker says:

    Cool project…I’m not sure how great it is for students since they didn’t show the math used to select components.

    • The Hackalizer says:

      True, that would have been nice. I guess you can derive the frequency values from the passives using the datasheet, but if you can do that you probably don’t need the lab exercise.

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