{"id":6441,"date":"2013-02-26T23:25:27","date_gmt":"2013-02-27T07:25:27","guid":{"rendered":"http:\/\/rustybolt.info\/wordpress\/?p=6441"},"modified":"2013-11-20T19:21:38","modified_gmt":"2013-11-21T03:21:38","slug":"2013-02-26-flasher-1-5v-switched-capacitor-from-te","status":"publish","type":"post","link":"https:\/\/rustybolt.info\/wordpress\/?p=6441","title":{"rendered":"2013-02-26 Flasher, 1.5V Switched Capacitor From TE"},"content":{"rendered":"

\"\"<\/a>I found this 1.5V switched capacitor flasher circuit on talkingelectronics.com (I can’t get a direct link to it).\u00a0 It flashes a red LED; I haven’t tried any other color but the similar Bowden circuit will brightly flash a yellow LED.\u00a0 This is essentially the same circuit I blogged earlier<\/a>, except this TE circuit uses only two transistors.\u00a0 My measurements show that it is oscillating at about 1.1 Hz, and draws less than 0.5 mA average current at 1.5V. The circuit is still (not brightly) flashing at 1V, and can be barely seen flashing at 0.8V.\u00a0 The red LED is what I’m measuring; other colors such as amber and yellow have a higher forward voltage and may flash brightly as long as the cell is fresh but may dim well before the cell is depleted.\u00a0 Several of the yellow ones I built have bright flashes, but I haven’t checked how they hold up as the cell is depleted.<\/p>\n

This circuit is essentially the flasher part of the Bowden circuit, without the third driver transistor.\u00a0 I’m assuming (but I don’t really know how accurate that assumption is) that the flash is not as bright as my Bowden flashers, because the current draw is not as high.\u00a0 I’m assuming this because the C2 in Bowden’s circuit is more than twice the capacitance, and R3 and R4 are less than half the resistance.\u00a0 However the Bowden circuit has the additional transistor; that and the power dissipated in R2 may reduce the difference.<\/p>\n

I made very few changes.\u00a0 The original circuit used a BC547 and BC557, which are not high current switching transistors.\u00a0 I used the ubiquitous PN2222A and the BC327-25 for the driver.\u00a0 TE recommended that R1 be from 100k to 470k.\u00a0 I have found that using a resistor lower than 1 Meg often causes this type of flasher circuit to stop oscillating when the supply voltage is at 1.5 or so volts.\u00a0 I usually recommend that this resistor be 1.3 or more Megohms.\u00a0 With that resistor, and a 1 uF capacitor it flashes about once per second.<\/p>\n

This TE circuit puts out a bright flash and draws very low current, giving long battery life.\u00a0 It’s simpler and cheaper than the Bowden circuit, but the difference should be accurately quantified so that the builder can make a judgment on whether or not the Bowden circuit has advantages that outweigh the added cost.\u00a0 Still, the experimenter can build this circuit and expect good performance from it with red, orange, amber or yellow LEDs.\u00a0 I tested one of the yellow LEDs for current and forward voltage.\u00a0 At 2.4V, the current was almost 30 milliamps, and at about 20 mA the voltage was about 2.3 volts.<\/p>\n

Update Feb 28 – Since the above, I’ve built a few more of this circuit.\u00a0 It’s a bit smaller than Bowden’s flasher, since it has one less transistor.\u00a0 I want to try experimenting with two flashers on the same board, driving a single LED.\u00a0 I would like to get the LED to look like it’s flickering, similar to a candle.\u00a0 This may require more than two flashers to get enough randomness to simulate the candle.\u00a0 I think when I have a few more single boards built, I may try to wire them together.\u00a0 More in the near future, I hope.<\/p>\n

Update Mar 2 – I wired two of the boards together.\u00a0 I removed the LED from one board and wired the junction where the anode was to the same point on the second flasher.\u00a0 I really wasn’t surprised when I found that the two flashers synchronized so there was only a single flash.\u00a0 The two\u00a0 flashers are coupled too closely, so that most likely the faster one triggers the slower one to fire early when the faster one fires.\u00a0 My idea has failed to produce the results I wanted.<\/p>\n

One thing, I haven’t tried experimenting with the 10 nF capacitor across the 1k resistor.\u00a0 Most circuits of this type that I’ve seen have no capacitor across the resistor.\u00a0 If there is one, it’s usually much smaller than 10 nF.\u00a0 But I have included it in every one I’ve built.\u00a0 I have a bag of a few hundred of them, so I figured that it was almost no cost to put it in the circuit.<\/p>\n

Update early Nov, 2013 – I have drilled and populated seven more boards for decorations during the Xmas holidays.<\/p>\n

Back to experimenting…<\/p>\n","protected":false},"excerpt":{"rendered":"

I found this 1.5V switched capacitor flasher circuit on talkingelectronics.com (I can’t get a direct link to it).\u00a0 It flashes a red LED; I haven’t tried any other color but the similar Bowden circuit will brightly flash a yellow LED.\u00a0 This is essentially the same circuit I blogged earlier, except this TE circuit uses only <\/p>\n

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