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Test your reaction times with a DIGITAL REACTION TIMER By JIM ROWE So you think your reaction time is pretty good. Cocky, eh? Well, you might be surprised. This little project will let you test your own or anyone’s reaction time and read it out accurately on a digital multi­ meter. The ‘Brake!’ stimulus is a large red LED, while the subject’s response can be sensed via a pushbutton, foot pedal switch or even an optical detector, set up to sense the light from a car’s brake lamp. E VERYONE TAKES a finite time to respond to any stimulus, whether it’s the brake lamp from the vehicle in front at 110km/h on the freeway, touching a hot saucepan on the stove or whatever. There’s the short time for the nerve impulses from your senses to travel to your brain, the time for your brain to respond and then a further short time for outgoing nerve impulses to travel to 56  Silicon Chip your limbs and stimulate the muscles to produce your reaction. These three delays are usually lumped together into a single quantity known as your reaction time: the total time taken for you to actually respond to such a stimulus. Your reaction time varies depending on whether you respond with your hand or your foot. It also depends on your state of health, alertness, psycho- logical outlook and whether you have recently taken drugs or alcohol. The reaction time for a normal healthy adult seems to vary from 150-300ms (milliseconds) for a hand response and from 400-800ms for a foot response (eg, hitting the brakes). If you are driving a vehicle and your measured reaction times are significantly longer than these times, you are an “accident waiting to happen”. www.siliconchip.com.au It’s designed to be a low-cost but accurate short-interval timer, suitable for a whole range of purposes (not only reaction). There is no case (cost saving #1), the push-button switches are mounted in old film cannisters (or anything else you wish – cost saving #2) and there is no output circuitry or display, because the output is read directly on any digital multimeter – cost saving #3. You don’t need to be a rocket scientist to work out why. Consider driving at 70km/h. At that speed, you’re travelling a distance of 19.4 metres every second or almost two metres in each 100ms. So if it takes you (say) 500ms to respond to an emergency by stepping on the brake pedal, your car will travel almost ten metres before the brakes can even begin to slow you down. Some safety experts have been lobwww.siliconchip.com.au bying for years to make reaction time testing mandatory for driver’s licence renewals. It hasn’t happened yet – but in the meantime you can measure the reaction time of all your driving friends, to judge whether they should be on the road or not . . . Uses a digital multimeter This new Reaction Timer uses a digital multimeter to read out the time in milliseconds; you just switch it to the 2V DC range. The unit runs from a 9V battery or DC plugpack. It measures the time you take to press the Stop button (or a foot switch) after the “Brake” LED is lit and converts that time into a DC voltage (1ms = 1mV). So your digital multimeter can read reaction times directly. A reading of 335mV corresponds to a reaction time of 335ms, and so on. June 2003  57 58  Silicon Chip www.siliconchip.com.au Fig.1 (left): the circuit uses a 1kHz clock pulse generator based on IC1c. Its pulses are gated through to binary counter IC3 (via IC2c) during the time that the “Brake” LED (LED 1) is illuminated. The counter outputs are then fed to a ladder DAC to produce an analog voltage for the DMM. Using a DMM for the readout keeps the circuit simple and the cost low. It also keeps the current drain low as well, so the tester will operate for quite a long time from a standard 9V battery. The current drain is only 4mA when the LED is not lit, rising to 14mA when the LED is on. Can you jump the gun? Nope. But you can have fun trying! To make it impossible to ‘jump the gun’ – even when you’re measuring your own reaction time – there’s a built-in variable time delay before the ‘Brake!’ LED is lit, after the Set button is pressed. So even if you press the Set button yourself, or notice when the operator presses the button, there’s no way of guessing when the LED will light. It could be anything from a fraction of a second up to a few seconds, before the LED lights and your reaction time begins to be measured. You are therefore forced to concentrate on the LED, and then push the Stop button as soon as you see it light up. The measuring range of the timer is from zero to 1023ms, or just over one second. If your reaction time is longer than this, the timer’s output voltage drops back to zero and starts again. This is hardly a problem though, because if your reaction time is longer than 1023ms you should probably be a passenger, not a driver! How it works At the heart of the timer is a simple clock pulse generator producing a string of pulses at a rate of one pulse per millisecond (ie, 1kHz). These clock pulses are controlled by a logic gate, which is opened only during the time that the ‘Brake!’ LED is illuminated. Pulses from the gate are then fed to a binary counter which counts how many pulses have been allowed through the gate. We then use a simple digital-to-analog converter (DAC) to convert the count into a DC output voltage, www.siliconchip.com.au ready for measuring by a DMM. That’s the basic idea. Now we can look at the circuit of Fig.1 in some detail. The 1kHz clock pulses are produced by the circuitry around IC1c, one section of a 40106 or 74C14 hex Schmitt trigger inverter. This is connected as a relaxation oscillator, with the 5kΩ variable resistor VR1 used to adjust its oscillation rate to exactly 1kHz. The pulses from IC1c are fed to gate IC2c, the main timing gate. IC2c is one section of a 4093 quad Schmitt NAND gate. The pulses which IC2c allows through are fed to the clock input of IC3, which is a 4040 12-stage binary counter. We only use 10 of the 12 outputs, as this allows us to count up to 1023 (one less than the 10th power of 2). Ladder DAC The 10 outputs of IC3 are in binary form, each one swinging between 0V and 5V as the counting proceeds. The combination of 10 binary outputs is converted into an equivalent analog DC voltage by the DAC ‘ladder network’ of 10kΩ and 20kΩ resistors. This simple but effective DAC ensures that each output is given the correct ‘binary weighting’ at the output. That is, the effect of each counter output halves with its position down the ladder. Output O8 produces half the output voltage of O9, O7 produces half that output again and so on. As this basic DAC produces an output voltage varying from 0V to just on 5V, we use the two additional 12kΩ and 3.3kΩ resistors connected from the DMM output to earth to form the lower half of a voltage divider. This reduces the output voltage range to 0 - 1.023V, ensuring that the DMM will read directly in millivolts. So IC1c, IC2c, IC3 and the resistor ladder network are essentially the core of the timer, able to count a time period and convert it into an equivalent DC voltage. Now let’s see how we make this timer measure reaction times. Gate IC2c is controlled by an RS flipflop formed from gates IC2a and IC2b (4093). When this flipflop is in the Set state with IC2b pin 4 high, gate IC2c is ‘open’ and allows 1kHz pulses through to the counter. At the same time transistor Q2 is turned on by the logic low at the output of IC2a (pin 3), via the transistor’s If you mount the pushbutton switches in a film cannister or similar, it’s a good idea to fit a large flat washer to stop the switch being forced through the plastic due to over-exuberance! 10kΩ base resistor. This turns on the ‘Brake!’ LED. This LED remains alight while the timer is actually measuring a reaction time, ie, until the person being tested pushes the STOP button. When the person being tested presses the Stop button (either S2, or a remote switch via CON2), this pulls pin 1 of IC2a low, which switches the RS flipflop back to its reset state. The output of IC2b goes low, turning off gate IC2c to stop the counter, while the output of IC2a goes high at the same time which turns off Q2 to extinguish the LED. But what switches the flipflop into the set state in the first place, to start the timer and light the LED? Now that’s a little more tricky – which is why we’ve left it until last. Random start delay The flipflop is switched into the set state by applying a brief logic low pulse to pin 6 of IC2b; we could do this by connecting the Set button S1 (or a remote switch via CON1) to this pin via a simple RC debounce circuit like that used for the Stop button S2. But this would turn on the LED and timer immediately, leaving the timer susceptible to errors caused by a subject “jumping the gun”. As a result, we’ve introduced a variable delay between pressing S1 and the actual turn-on of the flipflop, which “randomises” the turn-on procedure. This works as follows. Schmitt inverters IC1f and IC1e are both connected as relaxation oscillators, similar to the clock oscillator (IC1c) but with both working at much lower frequencies. IC1f runs at about 10Hz while June 2003  59 Parts List 1 PC board, code 04106031, 76 x 128mm 1 momentary contact pushbutton switch (S3) 2 momentary contact pushbutton switches (S1,2) OR 2 3.5mm PC-mount stereo jacks (CON1,2) 1 3.5mm PC-mount stereo jack (CON3) 1 2.5mm concentric power socket (CON4) 4 rubber feet, screw mounting type 4 M3 x 6mm machine screws with M3 nuts 1 3.5mm mono jack plug 1 1-metre length of light-duty figure-8 cable 2 banana plugs (one red, one black) 2 3.5mm mono jack plugs (optional) 2 2.5m lengths of shielded audio cable (optional) 2 pushbutton or foot switches (optional) 1 5kΩ horizontal trimpot (VR1) IC1e runs at around 8Hz, determined mainly by the 4.7µF capacitors and the 82kΩ or 100kΩ resistors. Both these oscillators produce an output in the form of very narrow negative-going pulses. This is due to the effect of the 1kΩ resistors and diodes D1 or D2 which make the 4.7µF capacitors discharge very rapidly on every half-cycle. So both outputs are at the logic high level for about 99% of o No. o   1 o   3 o   1 o   2 o 11 o   1 o   1 o 13 o   1 o   2 o   1 60  Silicon Chip Value 1MΩ 100kΩ 82kΩ 22kΩ 20kΩ 15kΩ 12kΩ 10kΩ 3.3kΩ 1kΩ 330Ω Semiconductors 1 40106 or 74C14 hex Schmitt trigger (IC1) 1 4093 quad Schmitt NAND gate (IC2) 1 4040 12-stage binary counter (IC3) 1 78L05 3-terminal regulator (REG1) 1 PN100 NPN transistor (Q1) 1 PN200 PNP transistor (Q2) 1 10mm bright red LED (LED1) 6 1N4148 diodes (D1-D6) 1 1N4004 power diode (D7) Capacitors 1 10µF tantalum 3 4.7µF tantalum 1 2.2µF tantalum 6 100nF monolithic (code 100n or 104) Resistors (0.25W 1%) 1 1MΩ 1 12kΩ 3 100kΩ 13 10kΩ 1 82kΩ 1 3.3kΩ 2 22kΩ 2 1kΩ 11 20kΩ 1 330Ω 1 15kΩ the time and only at logic low level for about 1% of the time. In other words, the oscillators have a very high duty cycle or mark-space ratio. Because the two oscillators are running at different frequencies, these narrow negative-going pulses coincide only occasionally. So by combining them in the AND gate formed by diodes D3, D4 and the 22kΩ resistor, we end up with a voltage across the resistor which is at logic high level most of the time, only occasionally going low very briefly. This becomes our source of pseudo-random pulses for triggering the flipflop. The occasional low pulses are inverted by IC1d and then fed to one input of NAND gate IC2d, which controls when they are allowed through to pin 6 of IC2b. The remaining circuitry using Q1, diodes D5 & D6 and inverter IC1b is used to ensure that the flipflop is switched to the set state on the arrival of the first ‘random’ pulse from IC1d after the Set switch S1 has been pressed. They also ensure that the flipflop can’t be retriggered again for some time, so that it switches to the reset state as soon as the Stop button is pressed, and remains in that state. This works as follows. While the flipflop is in the reset state, the output of inverter IC1b is high. This means that the 4.7µF capacitor connected between pin 12 of IC2d and 0V could potentially charge up to logic high via D6 and the 22kΩ resistor, except for the fact that transistor Q1 is switched on by the 10kΩ resistor connected to its base. But if the Set button S1 is pressed, Q1 turns off and the 4.7µF capacitor charges up rapidly, bringing pin 12 of IC2d to logic high level. IC2d then turns on, allowing the next ‘random’ pulse from IC1d to pass through to the flipflop and switch it to the Set state. Because of the high value of the 1MΩ resistor connected in parallel with the 4.7µF capacitor, the capacitor takes about 10 seconds to discharge when S1 is released. This means that you only have to press S1 briefly and the circuit remains ‘primed’ and ready Resistor Colour Codes 4-Band Code (1%) brown black green brown brown black yellow brown grey red orange brown red red orange brown red black orange brown brown green orange brown brown red orange brown brown black orange brown orange orange red brown brown black red brown orange orange brown brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown grey red black red brown red red black red brown red black black red brown brown green black red brown brown red black red brown brown black black red brown orange orange black brown brown brown black black brown brown orange orange black black brown www.siliconchip.com.au Fig.2: install the parts on the PC board as shown in this full-size wiring diagram and photograph. to let through the next trigger pulse from IC1d, even if this doesn’t arrive for a few seconds. But how do we prevent the triggering circuit from being able to turn on the flipflop a second time, after the Stop button S2 has been pressed? That’s the purpose of D5 and its series 10kΩ resistor, because they ensure that any charge on the 10µF capacitor is rapidly drained away as soon as the flipflop is switched on. When the flipflop switches to the Set state, the output of IC1b goes low, diofde D5 conducts and the capacitor discharges through the 10kΩ resistor in less than 100ms. Reset function When the timer’s flipflop is switched off by the Stop button (S2), counter www.siliconchip.com.au IC3 simply stops counting with its outputs remaining at the millisecond count that was reached. This means also that the timer’s DC output remains fixed, giving you as much time as you need to read the DMM and record the time reading. Reset switch S3 resets the counter to zero so you can perform another reaction time measurement. Associated with switch S3 is a 100kΩ resistor and a 100nF capacitor which form a ‘de-bounce filter’. This is followed by inverter IC1a which provides a positive-going reset signal for IC3 when the button is pressed. As well as being a de-bounce filter, the 100kΩ resistor and 100nF capacitor also form a ‘power-on reset’ circuit to reset IC3 as soon as power is connected to the circuit. Power for the circuit can come from a 9V battery or 9V DC plugpack. This is fed through diode D7 to prevent reversed-polarity damage and is then passed through 5V regulator REG1. Trigger options You have two options regarding the timer’s Set triggering. The simpler approach is to use on-board push-button S1 but this means that the person being tested will be well aware when you have ‘started the ball rolling’. The alternative approach is to fit socket CON1 instead of S1 and connect to it a remote pushbutton (or foot switch) via a length of shielded cable and a suitable plug. The remote pushbutton can be mounted in a film container or some other small case that can be handheld. June 2003  61 Fig.3: this is the full-size pattern for the single-sided PC board used in this project. It can also be downloaded from www.siliconchip.com.au/Shop/10/1976 This allows you to press the Set button out of the test subject’s sight (although, as we’ve said before, there is a random time period after this switch is pushed to prevent cheating!). The same two approaches are available for the Stop triggering, where you can again use either on-board pushbutton S2 or a remote pushbutton connected via CON2. In this case there’s also a third option; instead of connecting a simple pushbutton via CON2, you can connect a small optical sensor circuit, so the timer can be stopped by an optical signal of some kind; eg, the stop lamp of your car. In this way, you could simulate an actual braking situation (without the risk of a collision!). As shown on the circuit, the optical sensor can consist of a BP104 or similar photodiode, a 47kΩ resistor and a PN100 transistor. Putting it together Virtually all of the timer’s circuitry fits on a small PC board measuring 76 x 128mm and coded 04105031. The component overlay diagram is shown in Fig.2. The only off-board wiring consists of the cables running to your DMM and to a 9V battery or plugpack supply, plus those to the remote Set and Stop buttons if you elect to use them. The PC board assembly is intended to be used ‘as is’, supported by four small rubber feet. Before starting assembly, inspect the 62  Silicon Chip copper side of the PC board carefully and make sure there are no hairline cracks in the copper tracks, or solder or copper bridges shorting them together. Fix any defects. Then start by fitting the two wire links to the top of the board. One of these is just to the left of trimpot VR1, while the other is just to the left of IC2 and IC3. This second link should be made from a short length of insulated hookup wire. Next, fit the various connector sockets to the board: DC power socket CON4, DMM output socket CON3 and the optional sockets CON1 and CON2 for the remote Set and Stop buttons. Note that the PC board has holes and pads to match either type of commonly available board-mounting 3.5mm stereo sockets, so there shouldn’t be any problems. If you’re not fitting CON1 and CON2, you can fit push-button switches S1 and S2 instead, plus the Reset button S3, which goes at the front centre of the board. Note that S3 must be fitted with its ‘flat’ side towards the back of the board. This also applies to S1 and S2, if you fit them. Next you can fit trimpot VR1; you may also need to slightly enlarge the PC board holes before the pins will pass through easily. The board has holes to allow either common type of mini trimpot to be fitted. The resistors can be fitted next, using the colour codes in the parts list as a guide. If you’re not confident about reading the colour codes, use your DMM to check the resistor values. It’s also a good idea to fit the resistors with their colour codes reading in the same directions, to make checking and troubleshooting easier in the future. With the resistors fitted, you can fit the remaining low-profile parts: signal diodes D1-D6 (all 1N4148 or 1N914) and the polarity protection diode D7 (a 1N4004). Take special care to fit all of these diodes the correct way around, as shown in the diagram of Fig.2. If you don’t, the timer either won’t work at all, or you’re likely to get some very strange results... Once the diodes are soldered in place you can fit the small monolithic capacitors, and then the tantalum and electrolytic capacitors. Don’t forget that the tantalum and electrolytic capacitors are polarised, and must be fitted into the board with the correct polarity. You should find each one’s polarity clearly marked on its body, and the positive side is indicated on the overlay diagram to guide you. All that remains is to fit transistors Q1 and Q2, voltage regulator REG1, the 10mm LED and the three ICs. The main things to watch here are that you make sure to fit each one in its correct location and with the correct orientation as shown in the overlay diagram of Fig.2. REG1 is in the same type of TO92 package as Q1 and Q2, so don’t confuse them. Note that some 10mm LEDs don’t have a ‘flat’ moulded into their plastic pack, so the only easy way to check their polarity is by the longer length of their anode lead. Therefore, make sure you fit LED1 to the board with this longer lead on the side nearest IC3. We suggest that you solder the LED’s leads to the board pads with the bottom of the LED package only about 8-9mm above the board. This allows you to bend both leads forward by about 30°, so that the LED is tilted towards the front. Because all three ICs are of the CMOS type, it’s a good idea to take precautions to prevent them from being damaged by static electricity while you’re handling and fitting them. The best way to do this is by making sure that the PC board’s copper tracks, your soldering iron and yourself are all at earth potential for this part of the operation. To earth yourself, you can use a www.siliconchip.com.au conductive wrist strap, connected to an earthed water pipe via a length of flexible insulated wire. This also allows you to drain away any charge on the board copper by simply touching it before you fit the ICs. Once the ICs are fitted, the final step in the board assembly is to fit the board with small rubber mounting feet, using four M3 x 6mm machine screws and M3 nuts. You also need to make up a lead to run from the timer to your DMM. This should have a 3.5mm jack plug on one end and a pair of banana plugs at the other. If you use red/black colour coded cable for this lead and fit red and black banana plugs, this will make it easy to connect up to the DMM with the correct polarity every time. Mind you, most DMMs these days have auto polarity, so it’s not really a problem. If you’re using remote Set and Stop switches, you’ll also need to make up the remote switch leads. These can use single-core shielded wire for the plain pushbutton or foot-switch leads, fitted with mono 3.5mm jack plugs. You only need to use shielded stereo cable and a stereo jack plug for the optical Stop sensor, because the extra wire and jack connection are needed for the photodiode bias voltage. Checkout & calibration Your reaction timer should now be complete and ready for checkout and calibration. The first step is to connect it to a 9V battery or nominal 9V DC plugpack. Use your DMM to check the voltage at pin 14 of either IC1 or IC2, or pin 16 of IC3 (measured against board earth, such as the lefthand end of the two resistors between CON3 and CON4). You should read +5V at all three of these IC pins. The LED should not be lit but if you briefly press button S1, the LED should light soon afterwards – within a few seconds. If 10 seconds pass and the LED still hasn’t begun glowing, try pressing S1 again briefly. This should cause the LED to light within another few seconds. If not, you’ve probably made a wiring error. So remove the 9V supply and look for a reversed diode or transistor . . . Once the LED does light, try pressing Stop button S2. This should extinguish the LED immediately. If you have connected the timer’s output lead to your DMM, it should now indicate a steady DC voltage somewhere between 0V and 1.023V. If you then press the Reset button S3, the voltage should drop back to zero. Assuming the above checks are successful, your Reaction Timer is working correctly and all that remains is to calibrate it so that your reaction time readings will be accurate. This can be done quite easily, although you do need access to either a calibrated oscilloscope or a frequency counter. These days, many of the better DMMs incorporate a frequency meter. If you don’t have access to either of these instruments, you might have to simply set trimpot VR1 to the centre of its adjustment range and hope for the best. If you do have access to a calibrated scope or frequency counter, accurate calibration is a snack. All you have to do is connect the (high impedance) input of either instrument to either pin 6 of IC1 or pin 8 of IC2 and read the frequency of the square wave signal. Then adjust VR1 until the frequency reads as close as possible to 1kHz (1000Hz). That’s it. With the clock pulse rate set to 1kHz, the timer’s output voltage should be within 2% or better of the reaction time period in milliseconds. Camera shutter timer? While we haven’t tried it, we imagine that this circuit (especially the main timing oscillator, counter and DAC) would also be quite useful as a short interval timer – eg, for checking camera shutter speeds. Obviously the “random start” oscillators (IC1e, IC1f) would not be needed, nor would the “Brake” LED or its associated circuitry. One way to sense the “lens open” time would be to use a phototransistor or photodiode to detect light coming through the lens. Again, we must emphasise that we haven’t tried this but we would imagine the phototransistor could be used to simply control IC2c, which in turn would allow oscillator pulses from IC1c into the counter on “light” and stop them on “dark”. 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