Need help choosing a relay

Hi,

I'm currently in the process of designing a clap activated activated circuit. The circuit incorporates a 555 timer and is based upon a 555 timer monostable circuit , so that, using varying capacitors, the length of time for which the output is on can be varied.

The circuit works fine using an L.E.D. due its low power consumption: when you clap, the L.E.D. comes on for a specific amount of time (controlled by the resistor/capacitor network of the 555 monostable circuit).

However, I'd really need the circuit to turn a 9v motor on. If I just stick a motor in the place of the L.E.D., minus the 330 resistor, it just draws too much current, blowing up the 555 chip.

The simple answer is to use a relay, but the pin 3 outputs only 90 or so mA. All of the relays from www.rapidelectronics.com seem to have 10A or similar ratings.

I subsequently need help with choosing a relay that will work in the above circuit: it would ideally need to come from Rapid Electronics.

Please help!!

http://www.mikroe.com/en/books/picbook/7_05chapter.htm

The bottom half of the schematic has a transistor driver and relay hookup.

EDIT: or **broken link removed**
Look for flasher circuit on this page.

Find out the maximum current of the motor then use a suitable power transistor to drive it instead of a relay.
A resistor feeds its base from the output of the 555.
Connect a reversed diode across the motor to arrest its inductive voltage spike when it is turned off.

Thanks for the quick reply,

R:E: those links, the circuit is grounded: I'm unfortunately limited to a 9v poer-pack battery. I have tried using a pretty heavy duty transistor, but after a while it just got hot and tempromental, with the circuit packing in until it cooled down.

I'm unsure as to whether or not something like this:

**broken link removed**

would work. Ignoring the fact that it's 12v and DPDT, its "nominal coil current" is rated at 60mA: however, it also says rated 5A... Basically, I'm confused..

A transistor method like Audioguru would save you on battery drain. Get that relay coil out of the circuit.

If your transistor got too hot then it is not rated for the high current of the motor or its base current was too low.

How much is the max current of the motor? How much current when it is rev'd up and is running continuously?

The 12V relay won't work from only 9V. Its coil needs 60mA from 12V and its contacts can handle 5A. A power transistor like a TIP41 can switch 2A when fed by a 555. A darlington power transistor like a TIP120 can switch 5A.
The output from a 555 is 200mA max.

You don't need to ground your circuit. The earth symbol defines the 0V part of the circuit, just connect the points with the earth symbol to the -V side of the battery.

Thanks for all the help: I'll try and get my hands on a heavier duty transistor

The ratings for the transistor must match or exceed the ratings of the motor. Then the base resistor must be calculated to provide enough base current or it won't work again. Don't forget the reversed diode across the motor.

Try using a MOS-FET driven directly fom the 555 using a ...4K7 res or so. The MOS-FET requires only a small gate curent etc. and simplifyes the circuit. It also will disipate much less heat (if it's well chosen). In this case too USE A REV DIODE. The relay draws too much curent

A Mosfet is perfect. An IRF540 is common and inexpensive. It can switch up to 33A.

I've purchased two types of higher rated transistors (I can't remember the exact model number here - I'll try and add them later) but the circuit still doesn't work. When you clap the motor will come on for a second, or occasionally a few seconds before subsequently just stopping.

I've uploaded a creenshot of the schematic here:

**broken link removed**

So you can see the exact circuit I'm using (an L.D.R. is used in place of an electret microphone as croc clips doesn't have a microphone installed).

Any help greatly appreciated!

It is very hard to read. But looks like a RC in parallel with the LDR? Also, no base resistor. Is it and FET you are using, not a transistor? The 330 look low for going in parallel with the LDR, but again, it is too small for my old eyes.

I did a PIC LDR circuit I could upload, but I am running late for work. If you want me to upload that tonight, PM me and I will post it.

With only 9V for the supply (it must have enough current to start the motor) the output high voltage of the 555 is only 6.5V to 7V. Then the emitter high voltage of the power transistor is only 5.5V to 6V. Then the loss through the diode causes a voltage to the motor of only 4.5V to 5V.

Your circuit doesn't have a supply bypass capacitor to supply some current to the motor when it starts. Use 1000uF.

Instead if you connect the transistor as a switch and use a series base resistor and connect the diode across the motor then if the battery has enough current the motor will get abut 8V.

Instead if you use a Mosfet replacing the transistor then the motor will get about 8.9V.

How can you design a circuit without knowing the current used by its load (the motor)?
Why did you post your circuit without showing a transistor part number?
How can you use a battery that doesn't have a current rating?

Why don't you measure voltages to find out why the motor stops too soon?
Does the supply voltage or output voltage of the 555 drop too low? Are the transistor's pins connected backwards so that the 555 does all the work and therefore gets too hot?

Why did you post your circuit without showing a transistor part number?

Click to expand...

Sorry, I've got two tyopes of heavier duty transistors:

BD435

2N3055


I apologise for not posting these sooner: I didn't have access to the exact model no. at the time of writing.

Instead if you connect the transistor as a switch and use a series base resistor and connect the diode across the motor then if the battery has enough current the motor will get abut 8V.

Click to expand...

As you've already seen I'm not particularly strong at electronics, how should I do this on the circduited linked to above?

Without knowing the max current of your motor and without knowing if the battery can provide that much current then it is difficult to select a series base resistor for the transistor. Use 39 ohms then the output current from the 555 into the base of the transistor will be about 160mA and a BD435 or 2N3055 transistor will have a minimum output current to the motor of about 3A. The BD435 has an absolute max current rating of only 4A.

Here is your circuit with the transistor connected like a switch. I tried to make the schematic bigger but it gets fuzzy when bigger.