Simple analog pressure sensor
Here's a simple analog pressure sensor I read about years ago in "Robot Builders Bonanza". It's not an accurate pressure sensor since it's affected by temperature, humidity and excessive compression but it's very good at telling the difference between a gentle touch and a firm squeeze. Best of all you probably got some for nothing last time you bought IC's.
This is a piece of conductive foam that my last picaxe IC came in. when I skewer it with two multimeter probes its resistance was about 1K. When I squeezed it tight it dropped below 400R. I found you must skewer it with thick wire to get a good electrical connection as you can see in this blurred photo. The wire was 10A automotive wire that I tinned. Two pieces about 20mm long skewer the foam. Thinner wires are soldered to these skewers.
This is the schematic for this simple sensor.
I found that you wanted to keep the two skewers close together for good sensitivity so the piece of foam doesn't have to be very big.
My piece of foam had a resistance of about 1K when it wasn't squeezed and about 400R when squeezed tight so I picked a resistor that was halfway between the two values to get a reasonable voltage swing from my voltage divider.
The output can go straight to an analog input of your processor.
This might be a good (cheap) way for your robot to tell how hard it's gripping an object.
If your robot needs to know if it has bumped into something gently or collided violently then this bumper might be the way to go.
Two small pieces of foam connected in series will detect a collision no matter where the bumper is hit or wire two of the above pressure sensors, one each side if you want to know where the bumper was hit.