This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
Remove the lock and cut the mortise deep enough to admit a 3-volt battery lamp with a suitable socket attached. The lamp is then connected to wires which are concealed and run to a battery of three dry cells in the basement or other convenient place. A small push button is attached in the line and placed near the knob on the door. A small recess must be cut in the mortise so that the light from the lamp will shine directly on the inside of the plate over the keyhole. - Contributed by Armand F. Lamarre, St. Remi, Can.