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MORE FUN WITH THE PICAXE – PART 3 This circuit has HEART! The PICAXE circuit this month approximates animal breathing and heart beats to such an extent that it seems almost alive! T HIS PROJECT AROSE while discussing heart and breathing rates with a sports medicine workmate. It quite convincingly generates both “heartbeats” and breathing sounds that alter with temperature. Left in a darkened room, it could easily convince the gullible that it’s a robot taking a snooze! For the medics (and non-medics) amongst you, the three variables wide­ ly known as TPR (Temperature, Pulse, Respiration) are perhaps the most fundamental “what’s up with the patient” nursing vital signs measure. As an example, check your own pulse and breathing rates, both while exercising and relaxing. The “heartbeat” LED effect is quite entrancing, since it slowly increases in brightness to a maximum, then fades away again to darkness. A normal flashing LED of course just turns on and off , with no dimming action. The beating action here looks most eye-catching in comparison. It could even be used as a status light in a more professional application, perhaps to add a “human touch” to some otherwise bland piece of equipment. You may even feel more affectionate towards your photocopier by Stan Swan if it was fitted with one of these heatbeat circuits! Incidentally, while this is a quite simple, indeed simplistic, type of project, it does point towards some of the “grown up” uses for this type of circuit in the real world. Many devices use visual and aural indicators to help us humans quickly work out what they are doing – you can easily envisage this type of circuit being adapted for such a purpose. The sensor The sensor used – a negative temperature coefficient (NTC) thermistor – has a resistance that decreases as Even the one-eyed cat was convinced . . . it was fascinated by the breathing sound but couldn’t quite find the person to whom it was attached. 80  Silicon Chip www.siliconchip.com.au the temperature increases – and vice versa. This action is, of course, similar to an LDR as we used last month – (low resistance in bright light, high in darkness). However, thermistors have nothing like the rapid response or resistance range of an LDR, so the effects are somewhat slower and less dramatic. Typically, an NTC thermistor such as the 100kΩ <at> 25oC type used here (Dick Smith Cat. R-1895), shows a resistance of about 300kΩ near 0oC, reducing to about 30kΩ when warmed to 50oC. A suitable voltage divider network again exploits this so that a varying voltage from the thermistor is fed to the pin 1 I/O channel input – see Fig.1. Suitable juggling of the “top half” resistor to 15kΩ yielded some six discrete steps over a 0-50oC temperature range. If you use thermistors other than the 100kΩ <at> 25oC type specified here, you may have to alter this resistor – a resistance wheel greatly eases the fine tuning. Pulse width modulation The PICAXE-08 can output a Pulse Width Modulation (PWM) signal that effectively generates analog voltages (0- 5V) from digital inputs – in effect a simple Digital-to-Analog Converter (DAC). Rather than a neat train of fixed width pulses, a “noisy” jumble of 0s & 1s is produced instead. However, the overall ratio of highs to lows is as specified by the duty cycle. Quite elegant uses of this analog output can result, such as capacitor Fig.1: without wanting to sound repetitious, you can instantly see the similarities between this month’s circuit and the previous two: a thermistor replaces the LDR in the input “voltage divider”. charging (and refreshing) to a desired level but the “heartbeat” use here just illustrates the PWM action and syntax. (PWM will also be used in a later PICAXE circuit involving a small DC motor driven via a transistor). warm lamp or cup (try to keep water drops off the electronics of course) or by placing the unit in the fridge. Be sure to alert your family first though, to avoid “there’s something breathing in the fridge” concerns. The program Footnote Perhaps the most obvious program need is to prevent the LED action briefly halting while the piezo sounds. It’s rather like your heartbeats ceasing when you breathe! “08s” execute program instructions sequentially, so this may be hard to overcome, however. Ample memory space is left for your own tweaking, with any number of refinements possible! Wider temperature ranges can be organised by a Some users report PICAXE programming may be unreliable using fresh battery packs, since the upper 6V operating voltage may then be significantly exceeded. Removing a cell or two, so that only 4.5V or even 3V is supplied, seems to overcome this problem. See over for program listing. Once again, we’ve made a few changes (for clarity) from the PICnic box photo above to the Protoboard layout at right. Follow the layout and you shouldn’t go wrong! www.siliconchip.com.au April 2003  81 PICAXE-08 COMMANDS USED THIS MONTH: PWM PWM syntax takes the basic form – PWM pin, duty, cycles (duty & cycle can be program variables or constants). Pin refers to the PICAXE I/O output pin (0 ,1, 2, 4). Duty (0-255) specifies the analog level desired (0-5 volts). Cycles (0-255) specifies the number of cycles (~5ms) delivered. Example: PWM 1, 100, 8 refers to the pin 1 I/O channel, 100/255 duty, 8 cycles (ie, 100/255 = 39% duty cycle; hence 39% of 5V = 1.96V output). BASIC PROGRAM LISTING (This can also be downloaded from http://picaxe.orconhosting.net.nz/heartled.bas) ‘ Demo PWM “Heartbeat/breathing “ PICAXE-08 April.03 SiChip Ver 1.0 14th Feb.03 ‘ Best assembled & tested with solderless “PICNIK” box as detailed SiChip Feb.03 ‘ Refer http://picaxe.orcon.net.nz for background info & potential of PICAXE-08 ‘ Extra parts = 100k NTC thermistor (DSE R-1895),Red LED, 1 x 15k ,1 x 330 Ohm ‘ NTC can be moved off board, but water proof(epoxy/hot melt glue?) if outdoors ‘ New commands here = PWM , SOUND 255 (=hiss), ‘ Ref.PICAXE prog.editor.pdf help files,& BASIC Stamp 1 manuals for insights ‘ via Stan. SWAN (MU<at>W, New Zealand) => s.t.swan<at>massey.ac.nz <= ‘———————————————————————————————— ‘ Byte b0= NTC measure-increases as temp rises b1= loop counter 0-255 ‘ Variables b2= divided NTC measure (approx.= R) b3= “heartbeat” delay ‘ b4= loop counter 1-6 to give suitable pulse/respiration ratio ‘———————————————————————————————— ‘ Lines beginning ‘ are program documentation & could be ignored if need be. ‘ Program available for web download => http://picaxe.orconhosting.net.nz/heartled.bas ‘———————————————————————————————— heartbeat: ‘ LED/PWM thermistor resistance monitoring routine ‘ approximates human TPR heartbeat & breathing! for b4=1 to 6 readadc 1,b0 b2=b0/4 debug b0 ‘ cycle heartbeat loop so approx 10 breathes/min ‘ low res.read NTC value via 15k voltage divider ‘ sub zero temps may give b0=0 & beating ceases! ‘ divide for a conveniently smaller step value ‘ show variable NTC value(s) to attached PC VDU for b1= 0 to 255 step b2 pwm 2,b1,1 next b1 ‘ counter loop so LED has multiple PWM cycles ‘ PWM pin 2 LED one cycle increasing pulse width ‘ effect is a pleasing surging brightness increase for b1= 255 to 0 step -b2 pwm 2,b1,1 next b1 ‘ When warm b2 decreases so step less/beat faster ‘ PWM pin 2 LED one cycle decreasing pulse width ‘ gives a fading brightness instead of sudden off b3= 100/b2 *10 pause b3 next b4 ‘ “invert” NTC b2 value & limit to a useful range ‘ delay decreases as temp rises ‘ continue heartbeat loop until time for breath! sound 4,(255,80) pause 400 sound 4,(255,40) ‘ inhale = breathe in “hiss” approx. 1sec ‘ hold 400 millisecs (seems ~normal ?) ‘ exhale = breathe out “hiss” approx. 1/2 sec goto heartbeat 82  Silicon Chip ‘ repeat routine Some more references and parts suppliers 1. http://picaxe.orconhosting.net.nz – author’s enthusiastic PICAXE-08 web page. Some more references and 2. http://picaxe.orconhosting.net.nz/heartled. parts suppliers. . . & paste. bas – program listing to copy 3. http://www.healthmedialab.com/html/ president/mckin3.html – a fascinating 1901 TPR chart of then US President McKinley, recorded while he was hospitalised with gunshot wound complications. 4. SILICON CHIP, February & March 2003 – introducing PICAXE circuits. 5. www.cpemma.co.uk/pwm.html – typical of web sites explaining PWM. 6. Dick Smith Electronics stock 100kW/25oC NTC thermistors – Cat. R-1890 or R-1895. 7. Dick Smith Electronics have large (heart sized?) 10mm red LEDs – Cat. Z-4060. 8. Oatley Electronics (www.oatleyelectronics.com) and Microzed (www.microzed.com.au) now stock PICAXE-08 ICs and many accessories. PIC, PICAXE, mEL, ATOM, & Various Components ALL IN STOCK MicroZed Computers Tel: (02) 6772 2777 Fax: (02) 6772 8987 WebLINK: www.microzed.com.au www.siliconchip.com.au