Reflection Detecting Cart With No Semiconductors

This instructable shows you how to make a simple reflection detector with no semiconductors. You can see the toy working in the video. The video shows a summary of this article. However, I suggest you read this article if you do not have much experience with electronics.

The old PSpice simulation software does not have an LED component. Thus I modelled the LED component with three general-purpose diodes.

The supplies show that you can buy a few LEDs in case you accidentally damage them. However, this is not the case for LDR (light-dependent resistor) because it is expensive. Also, I specified 10 mm thickness LEDs but you can buy 5 mm thickness LEDs instead which cost less money.

Components: Large Light Dependent Resistor (LDR), 10 mm LED (display LED) - 2 (1 spare), 10 mm Bright LED ( Illumination LED) - 2 (1 spare), 10 kohm or 1 kohm potentiometer, piece of cardboard (10 cm by 10 cm (4 inches)), toy wheels - 4, 1.5 mm thickness metal wire - 30 cm (1 foot) (for the wheels), 1 mm thickness metal wire - 20 cm, sticky tape, foil (30 cm by 30 cm (1 foot)), big piece of cardboard (30 cm by 30 cm(12 inches)), insulated wire - 20 cm, 9 V battery harness, 9 V battery.

Tools: wire stripper, pliers.

Optional components: solder.

Optional tools: soldering iron.

The circuit is simple. It consists of light light-dependent resistor (LDR), illumination LED, and normal display LED all connected in series. The resistance of the LDR decreases with an increase in the amount of light that it receives. In bright sunlight, the resistance can fall to about 1 kohms. In the darkness, the LDR resistance could be as high as about 500 kohms. The light from the illumination LED is reflected from the foil into the LDR thus turning ON the display LED when the cart is placed on top of the foil.

Calculate the maximum LED current:

IledMax = (Vs - Vled1 - Vled2) / Rldr

= (9 V - 2 V - 2V) / 1000 ohms

= 5 mA

This current is very small for 10 mm LEDs that actually need 20 mA. You can solve this problem by connecting four LDRs in parallel instead of just one LDR. The minimum LDR resistance will decrease to 250 ohms, thus increasing the maximum current to 20 mA. However, this solution will cost a lot of money.

When the cart approaches the foil, more light flows into the light-dependent resistor (LDR). The LDR resistance falls and increases the series current. This results in a higher current for the illumination LED. The illumination LED produces more light that is reflected from the foil into the LDR, thus further reducing the LDR resistance and increasing the series current. Thus what we have here is a feedback loop that gradually increases the series current.

I used PSpice simulation software student edition version 9.1

Simulations show a predicted maximum LED current of 5 mA.

I made the circuit on a piece of cardboard with pliers and did not use the soldering iron. You can see the top of the cart in the first photo and the bottom in the second photo. You can see the LDR at the bottom near the illumination LED that provides the light for the sensor. This is especially useful in the darkness. I used a 10 kohm potentiometer but you can use 1 kohm potentiometer.

I attached metal wire reinforcements to maintain the curved shape of the cart. This is important because keeping the sensor very close to the foil will prevent the circuit from working due to the saturation of the sensor from the reflected light. You might try using bigger wheels.

I attached the wheels with masking tape. I struggled to insert the 1.5 mm metal wire into the wheels because the hole was very small. I ended up bending the wire. I had to use two pliers to complete this task.

Testing showed that the cart detector circuit was working.