Various 555 Timer circuits

Hello readers

The purpose of this article is to follow on from our explanation of the 555 timer IC by demonstrating some simple yet interesting, noisy and plain annoying uses of the 555. They are by no means that complex, and intended to help move theory into practice.

Button de-bouncer

De-bouncer? How does one bounce a button in the first place? Many years ago I bounced a button on the arcade Sonic the Hedgehog – hit it so hard it popped out and bounced over the table… But seriously, when working with digital logic circuits, you may need to use  a momentary button to accept user input. For example, to pulse a trigger or so on. However with some buttons, they are not all that they seem to be. You press them once, but they can register multiple contacts – i.e. register two or more ‘presses’ for what seems like only one press. This could possibly cause trouble, so we can use a 555 timer monostable circuit to solver the problem. In our de-bounce example, when the button is pressed, the output is kept at high for around half a second. Here is the schematic:

What we have is a basic monostable timer circuit. For my example the output delay (t) is to be half a second. The formula for t is: t=1.1xR1xC1. The closest resistor I had at hand was 2k ohms, so to find the required value for C1, the formula is rearranged into: C1=t/(1.1xR1). Substituting the values for t and R1 gives a value of C1 as 227.274 uF. So for C1 we have used a 220 uF capacitor.

Now for a visual demonstration of the de-bouncer at work. In the following video clip, the oscilloscope is displaying the button level on the lower channel, and the output level on the upper channel. The button level when open is high, as the 555 requires a low pulse to activate. The output level is normally low. You can see when the button is pressed that the button level momentarily drops to low, and then the output level goes high for around half a second:

Make some noise

As we know the 555 can oscillate at frequencies from less than 1Hz to around 500 kHz. The human ear can theoretically hear sounds between (approximately) 20 and 20 kHz. So if we create an astable timing circuit with an output frequency that falls within the range of the human ear, and connect that output to a small speaker – a range of tones can be emitted.

The circuit required is a standard 555 astable, with the output signal heading through a small 8 ohm 0.25 watt speaker and a 4.7 uF electrolytic capacitor to ground. The capacitor stops any DC current flowing to ground, without this we will overload the current-handling ability of the 555. (I couldn’t help myself by trying it without the capacitor – pulled 550 mA from the 555 before it stopped working…). To choose the values of R1 and C1 to emit out required frequency, the following formula is used: f (frequency) = 1.4 / {(R1 + [2 x R2]) x C1}. To cover the range required, a 100k ohm trimpot was used for R1. Here is the resulting schematic:

The input voltage can fall within the specification of the 555, however for optimum results a supply of between 5 and 9 volts DC should be used. In the following demonstration, we used a 9V supply. The purpose of the video is to learn the relationship between the tones and their frequencies. You can see the frequency on my old counter and hopefully hear the result:

Our next example is to create a  siren effect, using two 555 circuits – one for a low frequency and one for a high frequency. To determine the value for R1 for the low and high frequency, I used the previous circuit and chose two tones that were quite different, and measured the resistance of the trimpot (R1) at those frequencies. My R1 value for the ‘low’ tone is 82k ohm and 36k ohm for the ‘high’ frequency.

The switching between low and high frequency will be handled by a 4047 multivibrator – the Q and Q outputs will control NPN transistors. The transistors are used as switches to allow current to flow from the supply to the 555 high or low tone circuit. We use this method as the 4047 is not able to source enough current to drive the 555 circuits. Here is the schematic:

Don’t forget to connect pin 14 of the 4047 to supply voltage. This circuit has been tested with a supply voltage between 5 and 12 volts. As the supply voltage increases, so does the amplitude of the square wave emanating from the 555 output pins, which in turn in creases the volume of the siren. At 5 volts, the entire circuit drew only 20 milliamps. Speaking of which, you can listen to a recording of the output here. If you wish to alter the time for each tone, adjust the value of what is the 47k ohm resistor on pins 2 and 3 of the 4047.

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Part review – Linear Technology LTC6991 “Timerblox” low frequency oscillator

Hello Readers

Time for a new component review – the Linear Technology LTC6991 low frequency oscillator. This is part of Linear‘s Timerblox series of tiny timing devices. The full range is described on their web site. It is available in DFN or SOT-23 (below)  packaging. Our example for today:

The graph paper in the image is 5mm square, so the IC itself is tiny yet worthwhile challenge. Although reading the data sheet may convince you it is a difficult part to use, it is actually quite simple. This article will give you the “simple way”.

Once again I have lashed out and will hand-solder an SMD onto a SOT-23 board:

Messy, but it works. Moving along…

My reason for examining the LTC6991 was as a lower-power substitute to using a 555 timer to create a square wave at various frequencies. Normally I wouldn’t give two hoots about the current draw, as everything on my bench is powered from a lab supply.

However when designing things for external use, they are usually powered by a battery of some sort or solar – so the less current drawn the better. The bog-standard TI NE555 has a current draw (with output high) of between two and five milliamps (at 5V). Which doesn’t sound like much – but our 6991 is around 100 to 170 microamps at 5V. These figures are for the respective timers without an output load. You can source up to 20mA from the output of the 6991, and when doing so will naturally increase the current load – but still it will be less than our triple-nickel.

The LTC6991 offers a period range of 1 millisecond to 9.5 hours; which translates to a frequency range of 29.1 microhertz to 977 Hz, with a maximum frequency error or <1.5%. Only one to three external resistors are required to setup your timing requirements. For a more detailed explanation, please see the data sheet.pdf. The duty cycle defaults to 50% however this can be altered by using the IC in voltage-controlled period mode.

Linear have made using the IC very easy by providing an Excel spreadsheet you can use to make your required calculations, available from this page. For example, to create a 1 Hz oscillator, we enter our figures in as such:

and the macro returns the following details:

Very convenient – a schematic, the required resistors, and example timing diagram. I recreated this example, however not having the exact values in stock caused a slight increase in frequency – with Rset at 750k,  Rdiv1 at 910k and Rdiv2 at 180k my frequency was 3.1 Hz. Therefore to match the accuracy of the LTC6991 you need to ensure a your external components are close to spec and a very low tolerance. It produces a good square-wave:

now accepting donations for a new DSO

If you cannot use the exact resistor values recommended, use resistors in series or parallel to achieve the desired values. Don’t forget to measure them in real life if possible to ensure your accuracy does not suffer.

Pin one (RST) can be left floating for nomal oscillation, when high it resets the IC and forces output (pin six) low. As you can see, it is very simple to use especially with the provided spreadsheet. The required formulae are also provided in the data sheet if you wish to do your own calculations. Pulse width can be controlled with a fourth resistor Rpw, and is explained on page sixteen of the data sheet.

Although physically it may be difficult to use as it is SMD, the power requirements and the ability to generate such a wide range of oscillations with so few external parts makes the LTC6991 an attractive proposition.

The LTC6991 and the Timerblox series are new to market and should be available from the usual suppliers in the very near future such as RS and element-14.

As always, thank you for reading and I look forward to your comments and so on. Furthermore, don’t be shy in pointing out errors or places that could use improvement. Please subscribe using one of the methods at the top-right of this web page to receive updates on new posts. Or join our Google Group.

[Note - The LTC6991was a personally-ordered sample unit from Linear and reviewed without notification]

Otherwise, have fun, be good to each other – and make something!

Education – the RC circuit

Hello readers

Today we continue down the path of analog electronics theory by stopping by for an introductory look at the RC circuit. That’s R for resistor, and C for capacitor. As we know from previous articles, resistors can resist or limit the flow of current in a circuit, and a capacitor stores electric current for use in the future. And – when used together – these two simple components can be used for many interesting applications such as timing and creating oscillators of various frequencies.

How is this so? Please consider the following simple circuit:

When the switch is in position A, current flows through R1 and into the capacitor C1 until it is fully charged. During this charging process, the voltage across the capacitor will change, starting from zero until fully charged, at which point the voltage will be the same as if the capacitor had been replaced by a break in the circuit – in this case 6V. Fair enough. But how long will the capacitor take to reach this state? Well the time taken is a function of several things – including the value of the resistor (R1) as it limits the flow of current; and the size of the capacitor – which determines how much charge can be stored.

If we know these two values, we can calculate the time constant of the circuit. The time constant is denoted by the character zeta (lower-case Greek Z).

The time constant is the time taken (in seconds) by the capacitor C that is fed from a resistor R to charge to a certain level. The capacitor will charge to 63% of the final voltage in one time constant, 85% in two time constants, and 100% in five time constants. If you graphed the % charge against time constant, the result is exponential. That is:

Now enough theory – let’s put this RC circuit to practice to see the voltage change across the capacitor as it charges. The resistor R1 will be 20k ohm, the capacitor 1000 uF.

Our time constant will be R x C which will be 20000 ohms x 0.001 farads, which equals 20 (seconds).  Notice the unit conversion – you need to go back to ohms and farads not micro-, pico- or nanofarads. So our example will take 20 seconds to reach 63% of final voltage, and 100 seconds to reach almost full voltage. This is assuming the values of the resistor and capacitor are accurate. The capacitor will have to be taken on face value as I can’t measure it with my equipment, and don’t have the data sheet to know the tolerance. The resistor measured at 19.84 k ohms, and the battery measured 6.27 volts. Therefore our real time constant should be around 19.84 seconds, give or take.

First of all, here is a shot of the little oscilloscope measuring the change in voltage over the capacitor with respect to time. The vertical scale is 1v/division:

And here is the multimeter measuring the voltage next to a stopwatch. (crude yet effective, no?)

The two videos were not the most accurate, as it was difficult to synchronise the stopwatch and start the circuit, but I hope you could see the exponential relationship between time and voltage.

What about discharging? Using the circuit above, if we moved the switch to B after charging the capacitor –  and R2 was also 20k ohm – how long would it take to discharge the capacitor? Exactly the same as charging it! So one time constant to discharge 63% and so on. So you can take the graph from above and invert it as such:

How can we make use of an RC circuit?

I’m glad you asked. Consider the following circuit:

When power is applied, the capacitor starts to charge, and in doing so allows current to flow to the emitter of the transistor, which turns on the LED. However as the capacitor charges, less current passes to the base of the transistor, eventually turning it off. Therefore you can calculate time constants and experiment to create an off timer. However, a preferable way would be to make use of a 555 timer. For example, an RC combination is used to set the pulse length used in astable timing applications, for example using R1, R2 and C1:

(For more information on the 555 timer, please read this article)

Another use of the RC circuit is oscillating. Due to varying capacitor values due to tolerance, you most likely cannot make precision frequency generators, but you can still have some fun and make useful things. Here is a classic oscillator example – an astable multivibrator:

What is going on here? Here it is in action:

and here is one side being measured on the little scope:

We have two RC circuits, each controlling a transistor. When power is applied, there is no way to determine which side will start first, as this depends on the latent charge in the capacitors and the exact values of the resistors and capacitors. So to start let’s assume the left transistor (Q1) and LED are on; and the right transistor (Q2) and LED are off. The voltage at collector of Q1 will be close to zero as it is on. And the voltage at the base of Q2 will also be close to zero as C2 will initially be discharged. But C2 will now start charging via R4 and base of Q1 to around 5.4V (remember the 0.6v loss over the base-emitter junction of a transistor). While this is happening, C1 starts charging through R2. Once the voltage difference reaches 0.6V over the capacitor, Q2 is turned on.

But when Q2 is on, the voltage at the collector drops to zero, and C2 is charged, so it pulls the voltage at the base of Q1 to -5.4v, turning it off and the left LED. C1 starts charging via R1, and C2 starts charging via R3 until it reaches 0.6v. Then Q1 turns on, bringing the base of Q2 down to -5.4V – switching it off. And the whole process repeats itself. Argh. Now you can see why Arduino is so popular.

Time for a laugh – here is the result of too much current through a trimpot:

So there you have it – the RC circuit. Part of the magic of analogue electronics!

As always, thank you for reading and I look forward to your comments and so on. Furthermore, don’t be shy in pointing out errors or places that could use improvement. Please subscribe using one of the methods at the top-right of this web page to receive updates on new posts. Or join our new Google Group.

Otherwise, have fun, be good to each other – and make something!