Hacking an Old Slide Projector Into a Super-Fast Slide Scanner

The Raspberry Pi-powered slide scanner can digitize 50 slides in two minutes. Two thousand slides can be photographed in just 1.5 hours!

Cabe Atwell
16 days agoHome Automation / Robotics / Sensors

Tjaap transformed an old Voigtländer Spezial PM 35 A slide projector into an incredibly fast and impressive slide scanner. The project features a Raspberry Pi with an SD memory card and power supply, Raspberry Pi Camera Module V2, TIPI20-transistor, 1 kilo-ohm resistor, Neutral Density (ND) gel sheet, wire, breadboard, and a rubber band.

First, tjaap removed the projector lens. Then, he took out the plano-convex lens to reduce the light and heat’s impact on the camera. However, the light still affects the images taken with the Pi Camera, making them overexposed. To counter this, he placed a piece of ND sheet inside, which filters out more light.

Tjaap uses the Pi and Braun Novamat 515 AF’s remote wires to advance each slide in a tray. He connected the TIP120 Darlington transistor’s left pin to the Pi’s GPIO pin 24. Then, a 1K Ohm resistor was integrated to limit the current through the transistor. Next, he attached the middle pin to the projector’s switch wire and the right pin with a ground pin on the Pi, and the projector’s ground wire

A rubber band wraps around the camera case and projector lens to improve the camera’s rotation when it tilts. The projector has a small opening to bring out the camera module’s flat cable to the Pi. He pulled up the bracket, inserted the cable, and pushed the bracket down to connect the cable.

Tjaap created a Python script to enable the camera’s video for focusing and testing. One downside is that the Pi Camera doesn’t have autofocus. To solve this, he used a plastic knob to turn the lens about two turns counterclockwise, which produces very sharp images at 4 centimeters.

He wrote another script, called diascanner.py, which sends a signal through the GPIO port to the transistor for 0.05 seconds. The purpose of this script is to press the projector’s button. The scanner’s speed is determined by how quickly the projector changes slides: 2.4 seconds. This means an entire tray of fifty slides can be digitized in two minutes. Two thousand slides can be photographed in 1.5 hours.

To improve this slide scanner, Tjaap wants to use a conveyer belt to insert slide trays into the projector, one by one. It also needs sensors to detect when a new tray begins and when every tray is complete.

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