First 555 Monostable Circuit

[uncrocks]

I'm very new with pcb's and electronics in general. I've created a schematic for a circuit I'd like to build.



What I'm hoping it'll do is once the power is turned on, the output will go high for ~1 second, then remain low, so I went with a 10uF poly film cap and a 100k 1% resistor to get the timing (1.1*R*C). Looking at the schematic, does anyone have any suggestions or see any problems?

 

[BradtheRad]

Things appear shipshape.

My simulation worked as your plan describes. The trigger pin is pulled low on power-up. The output stays high for about 1 sec.

Assuming a 100k resistor provides sufficient current to turn on the transistor and energize the relay coil, you're good to go.

 

[betwixt]

I would make two comments:

1. The 100K resistor in the base of the transistor is *probably* too high in value. It depends on the relay specification which you have not supplied but a value of 10K or less would be more normal.
2. I would add a diode (1N4148 or 1N400x) across the relay coil, cathode end to +VCC. it should not normally conduct but as the transitor turns off and the relay coil current is stopped, it could produce a significant reverse poolarity voltage as it's magnetic field collapses. The diode would conduct in this case and protect the transistor.

In this kind of circuit where sudden changes in current occur, it might be a good idea to add another capacitor across VCC and GND close to the 555. 10uF will do, there is no need to use a film capacitor, a cheaper electrolytic will suffice. It helps by providing a local power reservoir so the 555 supply doesn't jump when the relay current stops and starts. Without it you are more prone to mis-triggering the monostable.

Brian.

 

[BradtheRad]

Betwixt's recommendations are all good ones.

 

[uncrocks]

First off, thanks for all the replies

And betwixt, so something more like this?

 

[jeffrey samuel]

the prime reason for the resistance at base to have base current in micro ampere and never more than that

 

[betwixt]

Exactly!

To explain a little more, most relays require a relatively large amount of current through their coils to activate them and all the current has to flow between the collector and emitter of the transistor. Ideally, when 'off' the transistor passes no current at all and in reality a silicon transistor like the 2N2222 will pass virtually nothing. When 'on' you want it to be a perfect conductor, rather like a mechanical switch in the on position so the relay can draw as much current as it needs without any restriction. The problem is that the amount by which the transistor conducts is dependent on the current flowing on it's base pin and with a resistor as high as 100K it would limit the current to a very low amount and the transistor would probably only partially conduct. The result would be the relay having inadequate current to operate reliably and also the transistor possibly heating up.

Brian.

 

[uncrocks]

One more question: would there be any benefit to eliminating the transistor and running the output on the 555 straight to the relay?

 

[betwixt]

It depends on the relay. Some versions of 555 have very limited output capability and may not be able to drive the relay directly. Some can only supply a few mA while others are fine at 200mA, you would have to check the manufacturers data sheet to confirm the type you have. A transistor will always work, that's why you rarely see a direct connection.

Leave the diode across the relay coil but take note of the polarity. The transistor inverts the signal from the 555 so that a high voltage on it's output causes a low voltage on the transistors collector and vice versa. For direct drive you would have to connect the bottom side of the relay to ground and the top to the 555.

Brian.

 

[uncrocks]

OK, I only asked because I saw another circuit that had the output going right into the relay. And one more, one more question ;p
What scenarios would I use MOSFETs instead of transistors and/or relays?

 

[betwixt]

MOSFETs have the advantage of not drawing gate current (except for the instant of switching), they are voltage driven rather than current driven. Provided you give them enough gate to source voltage, which in most cases starts at around 4V to 5V although it varies with type, they make pretty good switches. Their drawback is they share a common source pin between their switch (drain-source) and the control voltage (gate-source) so they cannot provide insulation like a relay can. In a relay, the contacts and the coil have no physical connection so you can, within specifications, switch any voltage, any current and any polarity, a MOSFET has to shwre a common connection at the source pin and are sensitive to the polarity you are switching. Both have their good and bad points.

Brian.

 

[jeffrey samuel]

the place where input current is very minimum we go for MOSFET instead of trannies