Light Alarm

Abstract

The objective of this project was to design and fabricate a printed circuit board that emulates a light alarm. The device sounds an alarm with a piezo electric buzzer when a Darlington phototransistor or a light dependent resistor (LDR) senses light.  It may be connected remotely at the end of two pieces of wire, where it needs to be powered by a 9 volt battery. The device’s logic is being controlled with a HEF14011B interconnect (IC) and the phototransistors or LDR sensitivity can be adjusted with a trimpot potentiometer. To simulate a drawer being opened a wooden chest box was constructed.

Project Description

The alarm sounds as soon as the wooden chest is opened and light falls on the Darlington phototransistor (MEL12) or the currently used PDV-P7002 light dependent resistor (LDR). The original Darlington Phototransistor was able to detect very low levels of light, although the LDR used in the final design is less sensitive, it functions practically the same. The Darlington phototransistor is typically ten times more sensitive than a normal phototransistor. And a phototransistor is typically 100 times more sensitive than a photodiode. (But the frequency of response is different - something we are not concerned with here.)  The Darligton phototransistor had burnt out during testing, because the polarity was reversed. A replacement for this part was difficult to find a recommended LDR was used. The LDR’s response time is a little slower than that of the MEL12, but it is not noticeable.  The PDV-P7002 are (CdS), photoconductive photocells designed to sense light from 400 to 700 nm. These light dependent resistors (LDR) are available in a wide range of resistance values. They are packaged in a two leaded plastic-coated ceramic header. It will not work as well as your eye in detecting light because the human eye is the most sensitive light detector there is, but it does a very good job.

Figure 2: CK1000 LIGHT ALARM circuit schematic generated with DIP-Trace software

For best results have the LDR pointed in the direction which light will come from. I constructed a wooden chest for demonstration purposes. The LDR is able to detect light very easily once the lid is opened. The kit is constructed on a single-sided printed circuit board (PCB) and the overall layout is slightly larger than the original kit, because I did not agree with some of the silk screen patterns used. The original silk screen had the holes for the through-hole resistor components on the interior of their silk screen patterns instead of their conventional exterior locations. Making this alteration made my PCB slightly larger. In dark conditions the resistance of the LDR is high; typically several mega-ohms, which virtually makes it an open circuit. In light conditions the resistance falls typically to around 30 ohms in bright sunlight, and current is allowed to flow through the circuit.

Figure 3: CK1000 LIGHT ALARM PCB diagram generated with DIP-Trace software

  

Figure 4: PCB without components (left) and with components (right)

Many practical uses of this property are possible. The circuit diagram shows that the circuit consists of three main stages:

1. Light Detection: In dark conditions the LDR is virtually an open circuit. No current flows through the potentiometer into the base of the BC557 transistor (Q2). Thus the transistor is OFF and the logic level on the input line to the HEF14011B is LOW. The potentiometer gives some control over sensitivity of the LDR, by adjusting its resistivity the rate at which current is dissipated can be changed and the amount of time it takes for sound to stop after no more light is detected is altered.

2. Logic Circuitry: The HEF14011B IC, also known as 14-pin dual inline package (14-DIP), is a quad NAND gate interconnect (there are 4 of them), 2 inputs, 2 outputs. It is wired up to oscillate when the input to it goes high, that is the BC557 transistor (Q2) turns on after light is detected by the LDR. The oscillating output from the HEF14011B IC turns the BC547 transistor (Q1) on and off. When Q1 is ON, the tank circuit (the choke and the piezo buzzer), charges up. When the Q1 turns OFF the piezo (which acts as a capacitor) discharges into the choke and makes a 'click' as it does so. Once the piezo is discharged the magnetic field around the choke starts to collapse and recharge the piezo, and the cycle repeats. This is called 'ringing'. Of course there are some resistive losses and the amplitude of the oscillation is slightly less with every cycle. However, the ringing does not decay away completely before the transistor turns on again and recharges the tank circuit fully.

3. Circuit Discharge Cycle: After the alarm has been operating and it is put back into dark conditions again then alarm will continue to sound for about 3 - 5 seconds. This is due to the 1uF capacitor (C1) and 4M7 resistor (R2) which keep the input to the HEF14011B HIGH. When the voltage falls below about 30% of the rail voltage (9V) the oscillator turns off. To decrease the delay reduction of the R2 resistor is necessary.

Circuit Board Assembly: Assembly is straight forward and components may be added to the PCB in any order. All components are through-hole that had been soldered by hand. The 14-DIP interconnect has an IC mount on the board in case it becomes damaged by an electrostatic discharge, thereby burning out the chip and maybe swapped.

  

Figure 5: Wooden chest without CK1000 LIGHT ALARM (left) and wooden chest with CK1000 LIGHT ALARM (right)

Conclusion

Conclusion: Being a student in mechanical engineering this project has taught me a lot about basic circuits function, basic circuit board design, and how fabrication of printed circuit boards is achieved. Designing the schematic and PCB using DIP Trace proved to be rather challenging especially without any prior knowledge of how the program operated prior to this experience. DIP Trace provides a versatile platform allowing the user to full customize the design based on the desired specifications. Once the entire library was developed, along with the footprint, placing the components in the schematic moved very smoothly. I really liked the auto arrangement function that DIP trace had for the footprint layout.

Designing the wooden box was rather simple and straight forward. Using the machine shop facilities at Cal Poly allowed for enough resources such that the construction of the wooden box was fairly inexpensive and only took a total time of 4 hours to complete.

At first the scope of this project seemed daunting and I did not know what to expect. The fact that I was allowed to choose whichever type of circuit board that I wanted, made this project fun. After having completed this project it gave me more confidence to possibly take on a similar project when working in the industry in the future.

Downloads

• Presentation (PDF)
• BOM (PDF)
• Schematics (JPEG)
• Layouts (JPEG)