The frequency probe enables ”listening in’ to the speed of a computer. It is, however, also suitable for use with other digital circuits because it makes high frequency audible so that signal can be conveniently monitored.
A 12-bit counter serves as a frequency detector. The signal measured in a computer or digital circuit is divided by 1024 and is output at pin 14. It is then used to control a transistor, T1, which in turn drives piezo buzzer Bz1
The scale factor has been chosen to convert MHz into KHz, so that clock frequency of say an XT computer will be heard as a shrill 8 KHz tone. If higher frequencies need to to be monitored scale factor of 2048 and 4096 can be obtained by connecting R2 to pin 15 or pin respectively. If an HCT circuit is used the measuring limit is some tens of MHz. For frequencies <4 MHz, a standard Type 4040 may be used: that has the advantages that the supply voltage need not be exactly 5 V. The supply connections and the probe are best made from flexible wire terminated into crocodile clips.