Hello, I am using a schematic that I found online to make a proximity sensor (LED turns on when motion is detected). I used a 555 timer, a PIR sensor and transistors. I would like to change the circuit so that the LED illuminates only when it is dark. I have attached the schematic, any help is appreciated!

 

9V battery or 9V DC brick? If battery, it will only work for few hours and battery will be drained by 555 timer.
Does the PIR works on 9V or 5 V?
Where is the light sensor ?

 

Thanks for your response. I will be using a DC brick. The PIR works on 9V and, as of now, there are no light-sensing components. I don't have experience with photoresistors and therefore not sure how to reverse the circuitry so the LED will only illuminate when it is dark AND recognizes movement. Thanks again.

 

What is the nature of the signal from the motion sensor?

What are you trying to accomplish with the transistors on the timer output?

You could use an LDR+transistor, or a comparator to hold the timer in reset when it's light.

 

The signal from the PIR sensor is a digital signal.

The transistors on the timer output are serving as a current driver. I have tried using an LDR but I am almost certain that I am not incorporating it into my circuit correctly.

 

The signal from the PIR sensor is a digital signal.

Is it a DC level as long as motion is detected, or does it give a pulse?

The transistors on the timer output are serving as a current driver.

A bipolar timer will source about 200mA. 33 ohms is a low value for a current limiter. How much current will the LED tolerate?

I have tried using an LDR but I am almost certain that I am not incorporating it into my circuit correctly.

Show what you tried.

 

The two transistors form a constant current circuit for the LED - about 18mA irrespective of the supply voltage - so that's good.
To keep the '555 reset during daylight you could use this circuit. Adjust the pot to set how dark it needs to be to turn on.

 

While I like Albert's suggestion and given the ways to go about this I would have likely used a comparator solution. I would use the dual comparator LM293. One merit is the comparator method allows for Hysterisis using feedback from the output. One occasional problem with a LDR is setting your threshold where as we go from light to dark or dark to light there can be some indecision in the circuit. An example would be light to dark and a cloud passes over changing the ambient light striking the actual LDR. I realize doing this would change your design but it may be worth consisderation. Both outputs from the comparator could drive a simple two transistor AND gate.

Ron

 

The two transistors form a constant current circuit for the LED - about 18mA irrespective of the supply voltage - so that's good.
To keep the '555 reset during daylight you could use this circuit. Adjust the pot to set how dark it needs to be to turn on.
View attachment 143452

Thanks, Albert. A couple of questions:

Would the remainder of my circuit remain the same?
What is the component labeled '104' that is connected to pin 2? Is that a cap, and if so, what value?

 

Thanks, Albert. A couple of questions:

Would the remainder of my circuit remain the same?
What is the component labeled '104' that is connected to pin 2? Is that a cap, and if so, what value?

Yes, the rest stays the same, only the reset pin wiring changes.
The 104 cap is the timing capacitor for the circuit I pinched the LDR reset circuit from. Ignore that.
104 is the value of the capacitor - 10 followed by 4 zeroes in Pf. 100,000Pf or 100nF

 

Albert, I am going to put it together now, I'll let you know how it turns out! Thanks again for your time and thoughts.

 

Yes, the rest stays the same, only the reset pin wiring changes.
The 104 cap is the timing capacitor for the circuit I pinched the LDR reset circuit from. Ignore that.
104 is the value of the capacitor - 10 followed by 4 zeroes in Pf. 100,000Pf or 100nF

Albert, you have saved the day! Thank you again.

 

Yes, the rest stays the same, only the reset pin wiring changes.
The 104 cap is the timing capacitor for the circuit I pinched the LDR reset circuit from. Ignore that.
104 is the value of the capacitor - 10 followed by 4 zeroes in Pf. 100,000Pf or 100nF

New issue, with the reset pin now connected to the LDR circuit, the LED stays illuminated until the LDR is exposed to light. When the reset pin was connected to VCC, the LED would stay illuminated for 30 seconds. Any ideas how to keep the LED on for 30 seconds with the LDR tied to the reset pin?

 

New issue, with the reset pin now connected to the LDR circuit, the LED stays illuminated until the LDR is exposed to light. When the reset pin was connected to VCC, the LED would stay illuminated for 30 seconds. Any ideas how to keep the LED on for 30 seconds with the LDR tied to the reset pin?

Can you measure the voltage on the trigger pin, pin 2 of the '555?
It should be the same as your VCC.

 

Can you measure the voltage on the trigger pin, pin 2 of the '555?
It should be the same as your VCC.

Pin 2 reads at 9V (VCC).

 

And the voltage on pins 6 & 7 is?

 

And the voltage on pins 6 & 7 is?

5V

 

And the voltage on pins 6 & 7 is?

5V

 

It should rise slowly up to 6V then drop to zero as the LED switches off.
The problem here is that the charge resistor is 1MΩ so your meter may be affecting the measurement.
When the LDR is illuminated that voltage should be zero then, in the dark, it should stay at zero until the trigger from the PIR arrives. Disconnect the PIR and see what that voltage does.

 

It should rise slowly up to 6V then drop to zero as the LED switches off.
The problem here is that the charge resistor is 1MΩ so your meter may be affecting the measurement.
When the LDR is illuminated that voltage should be zero then, in the dark, it should stay at zero until the trigger from the PIR arrives. Disconnect the PIR and see what that voltage does.

It rises up to about 5.5V but just remains there until I illuminate the LDR. It then drops to 0V until i trigger the PIR again.

After disconnecting the PIR, pins 6&7 climb up to about 4.5 V and remain there until I illuminate the LDR, and it then drops to 0V.