1966 Adding Machine


Fifty years ago, electronic calculators were things of science fiction, and even adding machines were expensive. The little circuit shown here filled a few of the gaps. It appeared in the September 1966 issue of the British Radio Constructor magazine.

According to the author, the device was built in response to a request by a friend who needed an adding machine to be used at motor car rallies in order to tally the number of miles covered.

It consisted of a telephone dial, a relay, and an electromagnetic counter, the type that increased by one every time a pulse was applied. The telephone dial had normally closed contacts, so the relay was necessary to convert to individual pulses. Also, the author noted that the telephone dial contacts probably couldn’t handle the current required by the counter.

The end result was that the counter increased by one for each pulse. So if you wanted to add 3+3, you would dial 3 twice, and the counter would count to 6.

Two-digit numbers could be accomplished by dialing zero for each 10. So to dial 34, you would dial zero three times, and then 4.

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