John Adrian Shepherd-Barron, OBE (23 June 1925 – 15 May 2010) was a British inventor, who led the team that installed the first cash machine, sometimes referred to as the automated teller machine or ATM.

Shepherd-Barron was educated at Stowe School, the University of Edinburgh and Trinity College, Cambridge.

He conceived the idea for a self-service machine dispensing cash whilst lying in the bath. He was considering the problem of bank opening hours, having turned up at a bank after closing time one day and found himself unable to withdraw money.

The first De La Rue Automatic Cash System (DACS) machine, called Barclaycash, was installed outside the Enfield branch of Barclays Bank in north London in June 1967. The first person to withdraw cash was actor Reg Varney, a celebrity resident of Enfield known for his part in a number of popular television series. An early deployment of this device outside of the UK took place in Zurich on November, 1967. It was called "Geldautomat".

The DACS machines used cheque-like tokens (which were guillotined to the size of a normal cheque inside the machine) which had been impregnated with a radioactive compound of carbon-14. The radioactive signal was detected by the machine and matched against the personal identification number (PIN) entered on a keypad. 

The short-range beta emission from carbon-14 could be easily detected, and he determined that the radiation hazard was acceptable as "you would have to eat 136,000 such cheques for it to have any effect on you".Initially, a PIN length of six digits was proposed; Shepherd-Barron tested this system on his wife, Caroline, but found that the longest string of numbers that she could remember was four.

As a result, four-digit PINs were chosen and as ATMs expanded across the globe, this became the world standard.

According to en.wikipedia