{"id":6584,"date":"2013-03-09T19:10:01","date_gmt":"2013-03-10T03:10:01","guid":{"rendered":"http:\/\/rustybolt.info\/wordpress\/?p=6584"},"modified":"2013-03-16T17:38:12","modified_gmt":"2013-03-17T00:38:12","slug":"2013-03-09-tbd","status":"publish","type":"post","link":"https:\/\/rustybolt.info\/wordpress\/?p=6584","title":{"rendered":"2013-03-09 Light Deactivated Joule Thief"},"content":{"rendered":"

\"\"<\/a>I built this up – tack soldered it together – before I drew the schematic.\u00a0 My intention was to make a circuit that was very low on power when the light deactivated it, and to use parts that the neophyte experimenter would already have.\u00a0 I wasn’t concerned about complexity or cost, but the three extra transistors, resistors and LED are relatively inexpensive.\u00a0 As I said, the experimenter may already have them so it wouldn’t be any added expense.<\/p>\n

Some experimenters connect a CdS photocell across the base to emitter of the Joule Thief transistor.\u00a0 This shunts all of the 0.9 milliamp base current from the 1k to negative when light hits it.\u00a0 Thus during the daylight hours, the circuit is still drawing nearly a milliamp, which is much less than the 50 to 80 mA during normal operation, but it is still substantial and would discharge the battery over a few months.<\/p>\n

My choice was to not use a CdS photocell because they are much more expensive and difficult to obtain than a LED,\u00a0 resistors and transistors.\u00a0 Because the LED puts out so little current, it must have more than one transistor to multiply the current up to enough to turn the other transistors on or off.\u00a0 Also, the sensor LED’s current output must be inverted so that it shuts off, rather than turns on.<\/p>\n

Another characteristic of a CdS photocell is that the resistance is somewhat linear with light hitting it.\u00a0 It doesn’t have a “knee” like a diode has.\u00a0 As the light hitting a LED gets brighter, the output voltage finally gets to the point where it will reach 0.6V and turn on the transistor.\u00a0 This helps make the deactivation more decisive.\u00a0 With the CdS photocell, the resistance could decrease in a bright ambient light, but the circuit could continue to run, and all the current through the photocell is just wasted.\u00a0 There is a way to change this, it’s called a Schmitt Trigger.\u00a0 But this just adds complexity to the circuit with no significant benefits.<\/p>\n

Circuit function<\/h4>\n

Normally the feedback winding would be connected to the positive, here it is connected to the collector of a PNP transistor.\u00a0 During normal operation with no light on the sensor, the sensor LED and Q1 are open, and the 470k turns on Q2 and its collector current turns Q3 fully on, so it is as if the feedback winding is connected to positive.\u00a0 The Joule Thief operates normally.<\/p>\n

When the ambient light hits the sensor LED, the small current generated turns on Q1, which shunts all of the current from the 470k to negative.\u00a0 Q2 turns off, and Q3 also turns off, and no longer lets current enter the feedback winding.\u00a0 The Joule Thief cannot operate without the current to the feedback winding, and it turns off and the LED goes dark.\u00a0 The amount of current drawn while the circuit is disabled is only microamps – just the current through the 470k.<\/p>\n

How well does it work?<\/h4>\n

The circuit works just fine.\u00a0 When there is no ambient light, the voltage drop across the Q3 is only .036 volts.\u00a0 That’s low enough to make it look like the feedback winding is connected directly to the positive.\u00a0 There is no discernible difference in the LED brightness when I short across the Q3 – if you short Q3 E to C, it’s just a regular Joule thief.\u00a0 The light sensor is not very sensitive and has to be held about a foot away from my white LED task light to turn it off.\u00a0 But this is very dependent on the type of LED used and the gain of the transistors, especially Q1.\u00a0 In my case I used an amber LED that did not put out much light, and I think that it would be more sensitive if the LED was a superbright red LED.\u00a0 The circuit will usually turn off when it’s outside during daylight, due to the light being much brighter than the indoor lights.\u00a0 There’s a lot of room for experimentation with different transistors and colors of the LED to get various light sensitivities.\u00a0 The JT is just a run-of-the-mill everyday conventional JT, so the LED’s light output is dependent on what and how it’s made.<\/p>\n","protected":false},"excerpt":{"rendered":"

I built this up – tack soldered it together – before I drew the schematic.\u00a0 My intention was to make a circuit that was very low on power when the light deactivated it, and to use parts that the neophyte experimenter would already have.\u00a0 I wasn’t concerned about complexity or cost, but the three extra <\/p>\n

(Read More…)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[],"class_list":["post-6584","post","type-post","status-publish","format-standard","hentry","category-joule-thief-smps-dc-dc"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/6584","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=6584"}],"version-history":[{"count":22,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/6584\/revisions"}],"predecessor-version":[{"id":6603,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/6584\/revisions\/6603"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6584"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6584"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6584"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}