• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

i need help with my monostable circuit!!

Not open for further replies.
Hi guys, my monostable circuit doesnt turn off my motor!
^ this is the schematic diagram for the circuit.
From pin 3, i connected a resistor to a relay and then to the motor, however the motor wont stay on when the button switch is pressed. Without the relay and the motor, the monostable circuit works with a led.
This is my connection for my circuit IMG_4072[1].JPG


Well-Known Member
Most Helpful Member
Motor is likely drawing too much current from the rather weak 9V battery. Try it with a well regulate power supply.


Active Member
Yes to the power supply, also 9V batteries tend to be avg 500mA for Alkaline types variable less for dry cell. Guessing the motor is the current user in that circuit and of the relay, 5V relays tend to be around 20-50mA ranges.

Also the motor GND return should be placed close as possible to the power source GND. The noise and current use by that motor could result in common line impedance increase, to say that the wire onto the common GND can increase resistance from the motor common wire placed between the 555 IC common and the final common GND point to power supply whilst the motor is on and running. The motor gets in the way of the IC current flow when on.

In that Breadboard layout the motor during operation running would be driving 555 pin 1 higher than supply common, as it would be applying more current out than the chip. At that time the pin 1 would not be properly connected to GND.


Active Member
try the circuit and see if it works at that point.

Also a small capacitor of around 22uF or 47uF to help reduce draw spikes from affecting the 555 IC's power supply, This capacitor would be located at the 8 and 4 pin connections, can use the upper right pin 8 for direct placement of the caps + pole.
And then the Capacitors common - pole to the the Bread boards common point.
Motor wont still wont stay on after moving it near to the power source. I have two capacitors now in my circuit i still need two small capacitors at pin 8 and 4?


Active Member
Clarification, to me a low range uF capacitor I tend to call small, A capacitor that is greater than the supply voltage would work. Essentially the capacitor at the 555 IC power supply would help maintain power to the IC when components begin to draw current from the supply lines.

I was stating you could use either pins 8 or 4 as they are both the 555 IC positive connectors. Pin 8 shows the most direct connection point for an capacitor on the Bread board. The upper right hand cornet of the IC shows some available space for an capacitor.


Active Member
Also if the motor only runs for a short time that is less than desired (I'm stating the obvious) the Current draw of the Bread board can be the over all problem, Solderless Bread boards tend to have common loss issues.

Increasing the resistor value of R1 from 51K to say, 100K if that value of resistor is available to use or any value above 51K shows longer motor operation, then the board is likely the issue.

A strong common GND point usually helps this, such as a Star Point or Star Ground method where all gnd points are as close as possible to the supply GND.


Active Member
I have checked something also, the 10K resistor on the Transistor, NPN type? as it is in current sink configuration for the relay. An 10K is too high a value to get an decent current on the Transistors Base, a lower value resistor to get at least 4mA at Transistor Base to operate the relays current requirement, however I am unaware of that Transistor you are using. And the Voltage range other than the 9V battery.

What is the now being used power supply voltage and current output rating? with the Voltage of the power supply and Transistor type, I could state a decent value resistor to get the transistor to fully switch on if it is a standard switching type NPN.


Active Member
I have made an error, that is an 300 Ohm not an 10K, apologies for that. That resistor value could be far too low for the Transistor.


Well-Known Member
Most Helpful Member
What is the value of C1?


Active Member
Just a check list on whats going on in the image of the circuit on the bread board.

The Relay in image shows 5V DC, then a resistor for the transistor using a value of 300R (Ohm) would place 13mA current on the Transistors Base,
the Transistor collector connected to the relays coil should sink the relays coil current/Volt requirement of the Unknown relay current value,
5 Volt types are generally not too high in current tho some can be. Data on the relay would help so I am acquiring that. Then the other end of the coil to + power supply,

:notes: Relay coil does not have an polarity requirement.

Relay Data for that series, included in PDF form.

The coil shows 70R resistance at 5V so these are not like the panasonic works type that I initially referred them to, that is my mistake!

The relay shows to consume a decent 71mA at 5V, the 300R resistor on the unknown sinking Transistors Base is adequate.

What is the Transistors ID? the one connected to the relay.
I am just wandering of the Transistors current capability.

The motors current requirement is still unknown.

Also what is the motors power in Volts requirement if known?

And the resistor for the Capacitor timing part of the 555 timers discharge pin 7, resistor R1 with an value shown in image of 51K Ohm, if the circuit is functional to an point, try to increase that value to an higher level and observe when run if the motor operates longer as a result,

What name is that 555 Timer? A Manufacturer. those pins look Texas instruments.

And also in the relay datasheet it shows some info, the relay image shows normally closed type, not open.
That means the relay could be doing opposite of what you want to happen when operating the motor.
= relay off, motor runs. Relay on, motor stops.
If I'm seeing the datasheet correctly. As it is showing a single pole double throw SPDT type.

The issue may be with the relay. However if that relay you have shows pins similar to the datasheets pins, then the relay can be used, it just needs to have the motor wired to the other relay contactor pin.

Now I am not fully aware of that relay completely, however referring to the image provided, the upper violet (purple) wire can be brought over to the pin at the to left hand side, then the motor would be powered when the relay is ON. If it is a normally closed type.

The violet (purple) wire to the relays other contactor pin via the image, avoiding the Transistors Collector pin that would cause the Transistors collector to sink all available supply current that can exceed the transistors current rating and be damaged.



Active Member
all tho that Violet wire in the image will not reach, may have to make another or shift to the lower left power rail,

also may want to add an extra Diode from the motors positive RED wire to the supply positive as the motor is an inductive component.

The Diode is the same connection method as the flyback Diode for the relay coil.

The Diode Anode to the motor RED wire at the relay and the Cathode to the power supply positive.

When the motor shuts down the internals are an wire wound inductor providing magnetic polarity to spool the motor, just like an relay just uses the magnetism to function a metal plate to operate the contactor (relay contacts).
both are Inductive spike capable.
The transistor is 2N3904B331.
The motor power requirement is unknown.
555 timer is 5BW2N6FE3 NE555P.

The relay i am using shows similar pins to the datasheets pins, so i can use the relay? Do I just need to move the violet wire to the left side of the relay to power the motor? AND can i just remove the transistor and replace it with a bare wire? Thanks.


Active Member
I would leave the Transistor in control of the relay as of 2 reasons,
1 the inductive spike of the relay even if an Diode is provided would still be better blocked by the Transistors Collector to Base for less chance of the initial flyback spike seeing the 555 IC pin 3.
2nd, The current requirement for the Transistor Base is 7.5 avg mA to drive an 70mA average relay, thus with current amplification method the Transistors Base current requirement of 7.5 avg mA to control 70 ranged mA current from Collector to the Emitter is less effort on the 555 IC's part.

As for direct wiring the relay to system power rails ( if that's what you mean) then yes, you can apply the relays coil to power rails to test out the Contact pins arrangement configuration until the desired operation is obtained, all regarding if the relay has 5 to 6 pins, 2 are dedicated to the relays internal coil, the contacts should be the outer most 4 via the datasheet.

I just would avoid letting the 555 IC, even if it can Source/Sink up to 200mA, it's the inductive spike that I would not want the IC "seeing" being best blocked again by the Transistors Collector when the Transistor is off, then the Diode providing an minor delayed effect before allowing the spike to traverse back to power supply through the Diode.

Also an capacitor on the power supply rail to help snub some of the rise in rail from the inductive return to help lower the spike affect on surrounding components as much. An 22uF to 100uF value that exceeds the voltage of the supply would be adequate for a start. Example, 5V supply an 16V Cap would be an avg range greater than if not all ready known. 12V supply = 25v cap and so on.

The 2N3904 Transistor can handle 200mA, so 125-150mA is that devices good rating zone, that Transistor is fine for driving that relay.

And also on the moving of the violet wire might just resolve the problem, if the issues have been understood clearly on all sides.
Not open for further replies.

EE World Online Articles