Sensor circuit, Flasher circuit

Thread Starter


Joined Jul 24, 2006
hi guys, i'm given a sensor circuit & a flasher circuit.

Sensor circuit

  • the LED will lights up when the LDR does not sense any light.

Flasher circuit

  • when connected to a power supply, both LED will flash. (example: LED.1 lights up while LED.2 dim. LED.2 lights up, while LED.1 dims.)
    for short, onli 1 LED WILL light up 1 time. at a intervel of 1 sec.

i'm given an assignment, where i'm required to connect both of the circuit using a relay.
the completed circuit should work something like this:
- power supply connected to sensor ciruit.
- sensor circuit connected to relay.
- relay connected to flasher circuit.
- when power-on, nothing will happed.
- it'll ONLY WORKS when e LDR in the sensor circuit activate. (sense no light)
- LED in sensor circuit will lights up, power passes through relay. (activating it)
- both the 2 LEDs in e flasher circuit flash.

i know it sounds like an easy job, but i'm new to these.
please guide mi along.
would be best if someone could actually let me know which point to connect.
need it ASAP.
i've attached the layout of both e circuits.
many thanks.



Joined Jan 10, 2006
OK, What happens to Q1 when the LDR resistance goes high (dark)..?
(Hint...the voltage on the base rises to above 0.6 volts and switches the transistor ON).
So, what happens to the collector of the transistor when it is switched on...??
(Hint, the collector is pulled to within 0.6 volts of the -ve rail)
SOOO, do you think this must put the remaining 4.4 volts across the R3/D1 combo, lighting the LED..??

OK, in summery, LDR resistance rises with darkness, causing the voltage at the junction of the voltage divider made by R1/R2 to rise, which switches Q1 "ON", pulling the Cathode of the LED "LOW" lighting up the LED.

Lets think about that. The LED and limiting resistor (D1/R3) combo has most of the supply voltage present across it after the transistor has switched On.

Now, where to put the coil of a 5 volt relay... (Hint, it should work just fine with 4.4 volts across its coil)

NB, it is also a good practise to put a Diode across the coil to suppress Back EMF when the coil is switched off. (Cathode to supply, Anode to the collector of the switching transistor..... OPPS, there is another hint).

I'm sure you can figure it out with that info. I'll leave you to work out how to use the relay contacts to switch the Flasher circuit On .... :)

Thread Starter


Joined Jul 24, 2006
but i still can't get it working, hope someone could give mi in a more detail explain.
maybe a layout would be great.

Thread Starter


Joined Jul 24, 2006
i'm sorry, but i'm very new to this.
can you please show me the correct layout?
can edit pic to show me?
many thanks.


Joined Apr 26, 2005
Let's review your requirement.

You want the sensor to control the relay so that the relay provides power to the timer when it's dark.

Q: Is the transistor conducting or not conducting when it's dark?

We ask ourselves this question because we need to know whether the relay is energized or not energized when we place it in the sensing circuit.

Q: How do we represent that relay with it's contacts schematically?
Q: What does common, normally open, and normally closed mean?
Q: How do we activate the timer circuit?

There's probably a couple of different ways we can do this. The obvious one is route the Vcc through the controlling device. Another would be to see what that "reset" does on the timer.

Answering those questions should clarify a few things in your mind.


Joined Jan 10, 2006
Got some questions for you...

** the 390 ohm resistor is limiting the current through the LED to around 6mA (assuming the LED has around 2 volts across it)... How did I come to this figure..? Why wont the relay energise with the resistor in series with the coil, and why did the LED dim ..??
** Where does the Flasher get it's supply voltage from..?? Why is shorting out the + and - of the flasher circuit with the relay contacts a Bad idea..?
** Where should the Back EMF diode go to protect the transistor from spikes when the relay is De-energised..?

BTW, not trying to be a pain, but learning from Homework is a Good thing... ;)

Thread Starter


Joined Jul 24, 2006
sorry for the late replys, guys.
got it up working fine.
thanks for yr help, finally get it done.
thanks again.