Simple Delay LED Light

Simple Delay LED Light

The circuit of this device is shown in Figure 1. It is mainly composed of a single-power low-power operational amplifier integrated circuit IC1, two low-power transistors, a microphone and a high-brightness light-emitting diode. When the microphone MIC receives external sounds of a certain intensity (including various noises), it generates an output voltage of corresponding intensity and adds it to the comparator IC1c. When this voltage exceeds the comparator threshold value, its output is high potential, making V1 conductive, and its output voltage is added to the comparator IC1b. Similarly, this voltage is higher than the threshold value of IC1b, and IC1b outputs a high potential to stimulate the power amplifier composed of IC1a and V2, thereby driving the LED to emit light. The preset delay length is determined by the time constant of the charge and discharge circuit composed of C2 and R6, and R7 and R8.
In this lighting device, a button switch can also be added to light up the LED and continue to emit light for a predetermined time. The circuit power supply is also very convenient, using 4 nickel-metal hydride (MiMH) power supplies. The whole device works intermittently, and the battery can be used for 2 months. The whole circuit is in standby mode, consuming 400 uA, and 24mA when the LED is on.

Working principle and parameter selection
From the circuit structure, the microphone MIC is connected between the input terminal and the ground wire through a wire. It is connected to the positive pole of the power supply through R1, and the AC component of the signal is coupled to the in-phase pin of the op amp IC1c through capacitor C1. The adjustment of the microphone sensitivity depends largely on the value of R1. It is recommended that the value of R1 be 15kΩ. For different brands and models of microphones used, the value of R1 should also be changed accordingly to achieve the desired sensitivity. When the maximum sensitivity is adjusted, when the door is opened and the air flow exerts pressure on the microphone vibration membrane, the LED should be triggered and light up. IC1 uses TLC274, and its pin diagram is shown in Figure 2.

C1 is used to isolate the DC component of the microphone signal, and resistor R2 is used to form a channel for the AC signal to the ground potential. Resistors R3 and R4 form a voltage divider to provide the threshold value of the voltage at the inverting input terminal (pin 9) of the comparator IC1c. However, the signal level generated by various stray sounds in the outside world exceeds this threshold value, and pin 8 of IC1c outputs a high potential, driving transistor V1 to turn on. As a result, C2 is charged quickly. The signal level at the non-inverting input terminal (pin 5) of the op amp IC1b rises exponentially. This process has the same effect as pressing the button switch S1. Because the switch is connected in parallel with V1. Since there is no series resistor on the charging path of capacitor C2. Therefore, even if the stray sound lasts for a short time, C2 can be quickly charged. The op amp IC1b constitutes a comparator, and resistors R7/R8 form a voltage divider, which determines that the voltage at the inverting input terminal (pin 6) is 20% of the power supply voltage. It is its threshold value (about 1V). In other words, when C2 is fully charged. It takes about 1.5 minutes for the voltage on C2 to drop to the threshold value after discharging through R6.
The last stage is a power amplifier composed of op amp IC1a and transistor V2, which controls the LED current (i.e. brightness). It can be adjusted by changing the value of R11 (Iled=0.44V/R11). IC1a uses the voltage across R11 to control the conduction of V2 in the feedback loop. This ensures that the luminous intensity of the LED remains unchanged when the battery voltage drops to the inflection point of its characteristic curve. Connector K1 is used for an external charger to charge the battery BT1.

Calculation of delay time
The duration of LED lighting. That is, the delay time before the LED turns off. See Figure 1. As long as the signal voltage on the non-inverting input terminal (pin 5) of the comparator IC1b is higher than the voltage on the inverting input terminal (pin 6), its output will remain high. The voltage level at the inverting input (pin 6) of IC1b is:
U _PIN6 =U _BT1 ×R8/(R7+R8)
The voltage on C2 is discharged according to the natural logarithmic function law:
U _C2 =U _BT1 ×e ^-t/R6C2
Rearrange the equation for t:
U _C2 /U _BT1 =e ^-t/R6C2
ln(U _C2 /U _BT1 )=-t/R6C2
t=-R6C2 × ln(U _C2 /U _BT1 )
UC2 and UPIN6 should be equal, and after substitution, we have:
t=-R6C2×ln [(U _BT1 ×R8/(R7+R8))/U _BT1 ]
Eliminate the battery voltage from the equation. This delay time depends only on the resistance and capacitance values:
t=-R6C2 × ln[R8/(R7+R8)]
Substitute the actual parameter values ​​of the components in the circuit. Then we get:
t = -10 ⁶ X 47 X 10 ^-6 X ln[10 ⁵ /(3.9 X 10 ⁵ +1 X 10 ⁵ )]
= -47 × ln(10 ⁵ /4.9 X 10 ⁵ )
= 75s (seconds)

Installation precautions
This production is for learning experimental courses. The circuit can be installed on a strip plug-in board. It is convenient to install, disassemble and debug. If it is actually used, a small printed circuit board can be designed. After debugging, install it in the box. Pay attention to the arrangement and installation of components. The IC1 integrated circuit should be equipped with a socket and positioned in the middle of the circuit board. The input and output parts of the circuit are installed on the left and right of IC1 respectively. Solder the three leads of the microphone, button switch and battery power supply on the board respectively. If the printed board is to be installed in the box, try not to have leads for the connection sockets of the button switch and the charger, but directly solder them on the corresponding pins of the board. The microphone shell is installed in the box. A hole should be opened on the box. So that sound waves can pass through. The LED is soldered on the circuit board. It is best to install a reflector. In addition, pay attention to the polarity of C2.