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HEARTMATE Build it & keep tabs on your t icker Got a treadmill or exercise bike to help you keep fit? How do you know whether you are overdoing it? Build this Heart Rate Monitor and stop yourself carking it. You can monitor your pulse beat rate to maintain it within certain limits. There is a timer to limit the duration of your exercise, minimum and maximum pulse rate buzzers and a recovery rate display. By JOHN CLARKE 28  Silicon Chip Y EAH, YEAH we know there are plenty of miniature heart rate monitors for people who want to jog but they don’t include all the handy features of the SILICON CHIP Heart Rate Monitor. It will display pulse rates up to 235 beats/minute and has a number of preset buzzers to help you in your exercise program. A Heart Rate Monitor is an essential piece of equipment when you are exercising as you can maintain your pulse rate at the desired level. Fig.1 shows target pulse rates for people aged between 20 and 70. The target range is the pulse rate needed in order to provide suitable exercise for the heart. For a 25-year old, this range is about 140-170 beats per minute while for a 60-year old it is typically between 115 and 140 beats per minute. It is important not to overdo it. If you begin to feel weak and light headed, stop immediately. The SILICON CHIP Heart Rate Monitor has an audible buzzer which can be set to sound if your pulse rate falls outside the target range that you set. And having exercised within the target range for a preset time, a buzzer will sound to tell you to have a rest. Feeling completely knackered? The SILICON CHIP Heart Rate Monitor will monitor your recovery. This is the time to recover to your normal resting pulse rate after the exercise period. If you are really fit, your recovery time will be quick and if you are not (like most of us), it will be a lot slower. The SILICON CHIP Heart Rate Monitor is housed in a small plastic case which can be mounted onto the handle bars of your exercise machine, be it an exercise bike, treadmill or whatever. It is powered from a 9-12V DC plugpack so there are no bat­teries to go flat. The 3-digit display uses 7-segment LEDs which are bright and easier to see than small LCD units. It has three pushbuttons to control its operation and a column of LEDs to indicate the current function or reading on the display. The pulse detector is a finger stall with an inbuilt in­frared LED and infrared detector. The finger stall is held onto your finger with a strip of Velcro (hook and loop). The Heart Rate Monitor shows pulse rates from 26 beats per minute to 235 beats per minute on its 3-digit display. If your pulse rate is below 26b/m you are either a lizard or you are dead. Fig.1: this diagram shows the target pulse rates for people aged between 20 and 70 when undertaking exercise. For a 25-year old, the range is about 140-170 beats per minute, while for a 60-year old, the target range is about 115-140 beats per minute. Either way, it will be displayed as “Err” on the display. Actually the most likely possibility is that the finger-stall is not properly attached to your finger. At the other end of the scale, measured pulse rates above the 235b/m limit will be displayed as three dashes (---). When the heart detector is working properly, the top-most LED in the LED column flashes in unison with each pulse. The 3-digit display is updated on every second pulse. When the pulse Main Features •  Ideal for use with exercise machines •  Adjustable timer with buzzer •  Visible pulse indicator •  Timer end buzzer •  Timer stops below minimum •  •  •  •  pulse beat rate Start/stop timer control Recovery display 1,2,3,4 & 5 minutes Error display for no pulse Minimum & maximum pulse settings for audible buzzer rate is being displayed, the second LED in the column is also lit. A piezo buzzer will momentarily sound if your pulse rate goes above or below preset values. The Timer setting can be displayed by pressing the rate/timer switch and this also lights the Timer LED in the column display. The SET switch is used to display the three presets. Press­ ing the SET switch selects the minimum pulse rate display and lights up the “Rate min” LED. The minimum pulse rate value can then be adjusted using the up and down pushbuttons. The inbuilt (default) setting is 130b/m. Once a new value is selected, it is stored in memory regardless of whether the power is on or off. Pressing the SET switch again lets you set the maximum pulse rate and also lights the “Rate max” LED. The inbuilt set­ ting is 160b/m. The SET switch will also display the timer setting and light the Timer LED. Then you can adjust the setting from one minute to 255 minutes (4 hours 15 minutes) in one-minute steps using the up and down switches. The initial setting is 30 minutes. To start the timing period, press the Start/Stop switch when the display is showing either the pulse rate or July 2001  29 The circuit is built on two PC boards which are stacked together inside a standard plastic utility case. Power is supplied by a 12VDC plugpack. current timer setting. The timer will count down from its preset value if the pulse rate is above the minimum setting and the LEDs in the column will begin chasing. When the buzzer sounds, a LED will flash to explain the warning. For example, if your pulse is racing above the set maximum, the buzzer sounds and the “Max rate” LED flashes. Similarly, if there is an error in detecting the pulse, the display will show “Err” and the “Rate min” LED will flash. This will stop the exercise timer. You can start the timer again with the Stop/Start switch but the timer will only begin counting down when the minimum pulse rate has been reached. The buzzer will also sound when the timer has counted down to zero. Recovery mode At the end of the timer countdown, the Heart Rate Monitor goes into the Recovery mode. For the next five minutes the dis­play will show REC, then the current timer value, then REC and the current pulse rate. The corresponding Pulse Timer LEDs will be displayed as each reading is shown. At one-minute intervals, the buzzer will sound and current heart beat rate will be stored in memory, referenced to the current minute. If there is an error in the reading, then the pulse value will be zero. At the beginning of the recovery 30  Silicon Chip period, the minute display will show “-0” to indicate that the timer is less than one minute into the recovery period. After one minute the display will show “-1” and then -2, -3, -4 and -5 at each successive minute. At the end of the five minutes, the display changes to show the pulse rates stored at each minute interval. The display will show REC, then “-0” and then the stored pulse rate. It then shows “-1” and the next stored pulse rate and so on. The display will continue to cycle through these values until either the unit is switched off or a switch is pressed. Pressing a switch will return the unit to the pulse mode. Circuit description Now let’s have a look at the circuit of Fig.2. IC1 is the PIC16F84 microcontroller which is the heart of the circuit. Three 7-segment displays DISP1DISP3 and a bargraph LED display DISP4 are driven directly from the RB1-RB7 outputs of IC1 via 150Ω limiting resistors. This is a multiplexed display ar­rangement with all the “a” segments on DISP1, DISP2 & DISP3 tied together and the same comment applies to the “b” segments and the “c”, “d”, “e”, “f” & “g” segments. The LEDs within DISP4 are tied to the “b”, “a”, “f”, “g”, “e” and “d” segments, respectively. The LED displays have their common anodes driven by tran­ sistors Q1-Q4 from the RA0, RA1, RA2 and RA3 lines of IC1. For example, if RA0 is brought low, transistor Q1 will be switched on to apply power to the common anode of the LEDs in DISP4. At the same time, a low output on RB1-RB7 will light the corresponding LED in the display. After DISP4 has been lit for about 1ms, the RA0 output is taken high and the RA2 line is brought low to drive Q2 and dis­play DISP1. The new 7-segment data on the RB1-RB7 lines is pre­sented to DISP1 for the next 1ms. Then the RA1 line is brought low to drive DISP3 and so on. Note that the “c” segment output from RB3 also connects to one side of the piezo transducer while the other side is driven by transistor Q1 via diode D5. However, the piezo transducer is driven only when DISP4 is on and only if the RB3 output is brought low at this time. If it is driven, it effectively Fig.2 (right): the circuit is based on a PIC microcontroller (IC1) which drives three 7-segment LED displays and a LED bargraph. IRD1 and RD1 form the infrared pickup – its output is processed by op amps IC2a-IC2d and used to drive the RB0 input of IC1 via transistor Q5. July 2001  31 Fig.3: follow this layout diagram to build the PC boards and complete the wiring. Note the orientation of switches S1-S3 – they are all mounted with their flat side to the left. Table 2: Capacitor Codes gets a 1ms pulse every 4ms, an effective frequency of 250Hz. Diode D5 isolates the transducer from the circuit if the RB3 line is high and Q1 is off. The 10kΩ resistor across the transducer discharges its capacitance between each forward pulse. The Set, Up and Down switches (S1-S3) are monitored by the RA4 input. These switches also connect to the RA2, RA3 & RA1 outputs respec- tively via diodes D2, D3 & D4. Normally, RA4 is held high via the 10kΩ resistor connected to the +5V supply. Depending on whether RA1, RA2 or RA3 is low when a switch is closed, IC1 will respond with a programmed action. Diodes D2, D3 & D4 are included to prevent the RA1, RA2 & RA3 lines from being shorted if more than one switch is pressed at the same time.  Value IEC Code EIA Code  0.33µF  334  330n  0.1µF  104  100n  .033µF   333   33n  15pF   15   15p IC1 runs at 4MHz due to the crystal (X1) connected to pins 15 & 16. This frequency is divided by four for the internal opera­ tion of the microprocessor. An internal counter further divides this by four and then by 250. The resulting signal drives the dis­ Table 1: Resistor Colour Codes           No. 1 7 2 6 1 3 4 1 7 32  Silicon Chip Value 2.2MΩ 1MΩ 100kΩ 10kΩ 2.2kΩ 1kΩ 680Ω 560Ω 150Ω 4-Band Code (1%) red red green brown brown black green brown brown black yellow brown brown black orange brown red red red brown brown black red brown blue grey brown brown green blue brown brown brown green brown brown 5-Band Code (1%) red red black yellow brown brown black black yellow brown brown black black orange brown brown black black red brown red red black brown brown brown black black brown brown blue grey black black brown green blue black black brown brown green black black brown The signal processing board is fitted with 15mm tapped spacers and is mounted in the bottom of the case by clipping it into the integral side slots. The display PC board is then secured to the tops if the spacers using machine screws. play multiplexing at 1kHz. Further division by 1000 provides us with a pulse once every minute which updates the timer. The pulse signal is applied to the RB0 input of IC1 which interrupts the program whenever this input goes high. Internally to IC1, there is a counter which counts how many 2ms intervals there are between two heart beat pulses. This number is then divided into 60,000 and the result is the pulse rate. For example, if the pulse rate is 60b/m (beats per minute) there will be a pulse every second. The duration of two heart beats will be two seconds or 1000 x 2ms. Dividing 1000 into 60,000 will give the correct result of 60 which is shown on the display. The actual update period for the pulse display is once every second heart pulse. So the update is every 2 seconds at 60b/m, once every second for 120b/m and so on. Detection of the pulse is via an optically-coupled pickup using an infrared LED (IRLED1) and a photo diode (IRD1). The infrared LED is powered from the +9V supply via a 560Ω resistor and its light shines into the flesh of your finger and is reflected off the bone. The pulsating blood through the vessels modulates the amount of light being detected by the infrared diode IRD1. Op amp IC2a is connected as a current-to-voltage converter for IRD1 which exhibits a varying reverse current in response to the changing light from the finger. The anode of IRD1 and the non-inverting input to IC2a is biassed at +4.5V. Pin 9 is the inverting input and the cathode of IRD1 connects to this via a 1kΩ stopper resistor. The gain of IC2a is set by the 1MΩ resistor between pin 8 and the cathode of IRD1 while the .033µF capacitor provides high frequency rolloff above 4.82Hz. This is to attenu­ate 50Hz mains signals which might otherwise be amplified. Note that there is also a 2.2MΩ resistor from the cathode of IRD1 to ground. This makes IC2a function as an inverting DC amplifier with a gain of -0.45 and this causes pin 8 to sit at about +6.5V. The resulting current through the 1MΩ resistor to the 2.2MΩ resistor is enough to provide a very The piezo transducer is secured to the bottom of the case using machine screws and nuts, while the DC power socket is mounted on the side. Make sure that none of the mounting screws can foul the PC board. July 2001  33 Fig.4: here’s how it all goes together inside the plastic case. The snap-in locators in the case guides hold the PC boards in place. small reverse bias current through IRD1 and thereby ensure that it works at maximum sensitivity. The output of IC2a is AC-coupled via a 0.33µF capacitor, giving a low frequency rolloff below 0.48Hz or the equivalent of 29 beats per minute. IC2b has a gain of 11, set by the 1MΩ feedback resistor and the 100kΩ resistor connecting from pin 6 to the +4.5V rail. The 47µF capacitor provides a rolloff below .03Hz. The .033µF ca- pacitor across the 1MΩ resistor gives the same high frequency rolloff as in IC2a. IC2c is almost identical to IC2b except that it includes a gain control, VR1. IC2c’s output is AC-coupled to op amp IC2d which is connected a Schmitt trigger, by virtue of the positive feedback applied by the 1MΩ resistor between pins 12 & 14. The output drives the base of transistor Q5 via a 10kΩ resistor to provide the The U-shaped metal bracket on the back of the unit allows it to be attached to the handlebars of an exercise bike or treadmill. 34  Silicon Chip pulse signal to pin 6 of IC1. Power for the circuit comes from a 9V or 12V DC plugpack via diode D1 and power switch S4, to feed two 3-terminal regula­tors. REG1 produces +5V while REG2 produces +9V. The 9V supply is divided by two series 1kΩ resistors to give a +4.5V supply to bias the inputs of IC2. Construction The Heart Rate Monitor is constructed on two PC boards. Board 1, coded 04107011 and measuring 105 x 62mm, contains the displays and microcontroller, IC1. Board 2, coded 04107012 and measuring 105 x 62mm, contains the amplifier circuitry for the pulse sensors. The two boards are stacked together and housed in a plastic case Fig.5: the mounting clamp details. It attaches to the base of the case using two M3 x 20mm screws and M3 nuts. Making The Infrared Pickup Sensor Fig.6 (left): how the infrared pulse sensor is made. The infrared transmitting and receiving LEDs are fitted side-by-side into two slots that are cut into a PVC saddle clamp section. Below: the pulse sensor assembly is fitted with a length of Velcro so that it can be held in position on your finger. You can use contact adhesive to secure the Velcro to the PVC section. measuring 130 x 67 x 44mm. The full wiring details for both boards are shown in Fig.3. Begin construction by checking the PC boards for shorts between tracks or any breaks in the copper connections. Compare the patterns with the published artwork to be sure they are correct. Check hole sizes. The corner mounting holes and regula­ tor tab mounting holes should be 3mm in diameter. The holes for the PC stakes should be drilled to give a tight fit for these. You can work on both PC boards together. Insert the PC stakes first, followed by the links and resistors. Use the resis­tor colour codes in Table 1 when selecting the resistors and use a digital multimeter to check each one before it is installed. Next, insert and solder in the diodes, making sure that they are oriented correctly. Note that D1 is a 1N4004. The 7-segment displays July 2001  35 Parts List 1 PC board, code 04107011, 105 x 62mm 1 PC board, code 04107012, 105 x 62mm 1 front panel label, 125 x 63mm 1 plastic case, 130 x 67 x 44mm 1 transparent red Perspex or Acrylic sheet, 56 x 18mm x 2.5mm 1 DC panel socket (plus self- tapping screws if required) 1 9V or 12V DC 300mA plugpack 3 snap-action keyboard switches (S1,S2,S3) 1 miniature SPST rocker switch (S4) 1 4MHz parallel resonant crystal (X1) 1 piezo transducer 1 18-pin DIL socket 4 M3 x 15mm tapped standoffs 10 M3 x 6mm screws 2 M3 x 20mm screws 1 M3 x 6mm countersunk screw 6 M3 nuts 2 M2.6 x 15mm screws 2 M2.6 nuts 1 small rubber grommet 1 crimp eyelet with 3mm eyelet hole 13 PC stakes 1 25.4mm saddle clamp 1 200mm length of 25mm wide hook and loop tape (Velcro) 1 20mm saddle clamp (used for conduit) 1 10mm length of 12.5mm diameter heatshrink tubing 1 120mm length of 0.8mm tinned copper wire 1 50mm length of red medium duty hookup wire 1 50mm length of black medium duty hookup wire are inserted with the decimal point facing toward the switches. DISP4 should be inserted with the label side towards IC1. Insert the socket for IC1 with its pin 1 oriented as shown in Fig.3. IC2 can be soldered directly into the PC board. Now insert the capacitors. The electrolytic types must be oriented correctly with the positive side placed as shown on the overlay diagram and with each one laid over on 36  Silicon Chip 1 50mm length of green medium duty hookup wire 1 800mm length of single core screened cable (small diameter type) 1 100kΩ horizontal trimpot (VR1) Semiconductors 1 PIC16F84P microprocessor programmed with HEART.HEX (IC1) 1 TL074 quad op amp (IC2) 3 LTS542A 7-segment common anode red displays (DISP1DISP3) 1 DIL 10-LED (red) bargraph (DISP4) 1 photo interrupter for IRLED1 (Jaycar Cat. Z-1901 or equivalent) 1 IR photo-diode BP104, BP104, (IRD1) 1 7805 5V 1A regulator (REG1) 1 7809 9V 1A regulator (REG2) 4 BC328 PNP transistors (Q1-Q4) 1 BC338 NPN transistor (Q5) 1 1N4004 1A diode (D1) 4 1N914, 1N4148 switching diodes (D2-D5) Capacitors 1 100µF 16VW PC electrolytic 2 47µF 16VW PC electrolytic 5 10µF 16VW PC electrolytic 3 0.33µF MKT polyester 2 0.1µF MKT polyester 3 .033µF MKT polyester 2 15pF NP0 ceramic Resistors (0.25W, 1%) 1 2.2MΩ 3 1kΩ 7 1MΩ 4 680Ω 2 100kΩ 1 560Ω 6 10kΩ 7 150Ω 1 2.2kΩ its side, as shown in the photographs, to allow the PC boards to be stacked. For the same reason, the crystal is placed on its side and is secured at its free end using a short length of tinned copper wire soldered to the PC board. When inserting the pushbutton switches make sure that the flat sides are oriented as shown. The four transistors Q1-Q4 should be inserted so that their tops are level with the top of the dis­plays. Q5 is the BC338 and it does not need to be inserted so far into the PC board. REG1 & REG2 are mounted horizontally and the tabs are secured with an M3 screw and nut. The boards stack together as shown in Fig.4, with 15mm tapped spacers and M3 x 6mm screws. The integral side slots in the case must be cut away for the first 13mm to allow the assem­ bly to slide into place. Drill a hole in the end of the case for the DC power socket and drill another hole at the other end for the rubber grommet required for the pulse sensor leads. Use the front panel artwork as a guide to drilling the holes for the switches and to make the display cutout. The cutout is drilled and filed so that the red Perspex or Acrylic window is a tight fit. Attach the front panel label and cut out the holes in this with a sharp knife. Drill two holes to mount the piezo transducer on the base of the case and drill a central hole for the sound to escape. The details of the mounting clamp are shown in Fig.5. It attaches to the base of the case with two M3 x 20mm screws and M3 nuts, as shown in Fig.4. Sensor details Fig.6 shows how the pulse sensor is made. A section is cut from a 20mm PVC conduit saddle clamp and two slots cut into it install the IR sensors. The IRLED is taken from the IR interrupt­er assembly by carefully breaking the plastic housing in which the LED is secured. The IR LED is the one which has the diode symbol embossed on the top of the plastic housing. You can break the housing carefully with pliers and side cutters to release the LED. We do not use the detector within the housing since this is a photo transistor and is not suitable for detecting the small light changes involved. Cut the saddle clamp as shown in Fig.6 and mark, drill and file the rectangular slots for IRLED1 and IRD1. IRD1 should be able to pass through the hole so it is flush with the inside surface of the saddle clamp. The IRLED is best positioned so the rear face of the package is flush with the outside of the clamp. We used an eyelet as a cable clamp for the wires and this is attached with The pulse sensor should be wrapped firmly around your finger, to ensure a reliable pickup. Note that the sensor should go over the fleshy part of the finger, not over the bone as some bonehead has shown here. a countersunk screw which taps into the PVC material. Wires can be soldered to the IRD1 and IRLED1 leads after passing them through the eyelet crimp end. We used some spaghetti sleeving to protect the wires from the clamp. The leads are then secured in place with a 10mm length of 12.5mm diameter heatshrink tubing. Testing Connect the DC plugpack to the socket and check that there is a nominal 5V between pins 5 and 14 of the socket for IC1. There should also be 9V between pins 4 and 11 of IC2. If these voltages are correct, kill the power and insert IC1 into its socket. Make sure it is in the correct way. Apply power again and check that the display lights and the pulse rate LED lights. The pulse LED should flash on and off if you rapidly move your finger onto and off the sensor assembly. Placing the end of your finger over the sensor should make the pulse LED flash on and off (in time with your pulse) and a plus rate should be shown by the display. You may need to adjust VR1 to obtain sufficient sensitivity or it may need to be turned back for best results. Too much sensitivity can make the display show a higher figure than it should. You can set the minimum and maximum pulse rates using the graph of Fig.1 as a guide. The timer is set by selecting timer and adjusting the minutes displayed. Setting the timer to 60 will provide a timeout after 1 hour. You cannot set the timer to a value that’s less than 1 minute. Fig.7: here are the full-size etching patterns for the two PC boards. When the start switch is pressed, the value in the set timer will be transferred to the timer and it will begin counting down every minute. The three lower display LEDs in the bargraph should chase each other when the timer is counting down unless your pulse heart beat is above or below the preset maximum and minimum figures. You will need to make a finger stall with Velcro to hold the pulse sensor securely onto your finger while you SC exercise. Fig.8: this full-size front panel artwork can be used as a drilling template. July 2001  37