Would it be OK to say "put a little/small person in the machine"? If not, please kindly suggest a better replacement so that we can learn. :)
> When we surveyed our community about the usage and overall impact of the word “midget”, over 90% of our members surveyed stated that the word should never be used in reference to a person with dwarfism.
From https://www.lpaonline.org/the-m-word which is a dwarfism organization. But as other comments stated it's best to not hinge your metaphors on ridiculing others.
For many people there is one characteristic that strangers always notice or call them out on and seeing that trait brought up in odd metaphors or jokes would likely feel off.
I think the actual reference might be to a Calvin and Hobbes strip, but adding that it's a little person in the atm vs just a person -- an unnecessary addition.