{"id":4823,"date":"2012-10-31T19:19:01","date_gmt":"2012-11-01T02:19:01","guid":{"rendered":"http:\/\/rustybolt.info\/wordpress\/?p=4823"},"modified":"2012-12-14T12:57:04","modified_gmt":"2012-12-14T20:57:04","slug":"2012-10-31","status":"publish","type":"post","link":"https:\/\/rustybolt.info\/wordpress\/?p=4823","title":{"rendered":"2012-10-31 Sound Level Meter"},"content":{"rendered":"<p>The <strong><a href=\"http:\/\/www.zen22142.zen.co.uk\/Circuits\/Testgear\/soundlevelmeter.htm\">schematic found here<\/a><\/strong> looks to me like it could be a good sound level meter with a few changes.\u00a0 One thing that it has going for it is the meter is inside of the feedback loop, so it will have a linear scale at low levels.\u00a0 Some meters don&#8217;t and the forward voltage of the rectifiers causes the scale to be distorted at low levels.<\/p>\n<p>One problem it has is the coupling between the microphone and the opamp.\u00a0 The schematic shows a polarized electrolytic capacitor as the coupling capacitor.\u00a0 The problem is that every electret condenser microphone will have a different voltage drop across it when connected to this circuit.\u00a0 One ECM might have 4.5 volts or more across it, which would be ideal, since there is 4.5 volts on the negative side of the coupling cap and the cap would be correctly polarized.\u00a0 But another ECM might have less than 4.5 volts across it, which means that the cap&#8217;s positive terminal would be negative in relation to the other end, and the cap would be polarized incorrectly.<\/p>\n<p>The easy solution is to replace the capacitor with a non-polarized capacitor such as a 1 uF plastic or disk capacitor.\u00a0 Then the polarity doesn&#8217;t make any difference.\u00a0 But there&#8217;s a much better way to do this.<\/p>\n<p>Just remove the two 10 k resistors R1 and R2.\u00a0 Connect one end of a 100k resistor to the junction of R3 and C2.\u00a0 Connect the other end to the junction of R5, C1 and the ECM.\u00a0 When the circuit\u00a0 is first turned on, the R5 will supply a small amount of current to charge up C3 to the same voltage as that at the ECM.\u00a0 Since the opamp is a FET opamp, the inputs don&#8217;t require any current and the voltage across C1 will be zero.\u00a0 This saves one resistor, and the circuit can still use an inexpensive electrolytic for C1.<\/p>\n<p>Back to experimenting&#8230;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>The schematic found here looks to me like it could be a good sound level meter with a few changes.\u00a0 One thing that it has going for it is the meter is inside of the feedback loop, so it will have a linear scale at low levels.\u00a0 Some meters don&#8217;t and the forward voltage of<a href=\"https:\/\/rustybolt.info\/wordpress\/?p=4823\"> <\/p>\n<p> (Read More&#8230;)<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[8,17],"tags":[],"class_list":["post-4823","post","type-post","status-publish","format-standard","hentry","category-audio","category-meters-and-test-equipment"],"_links":{"self":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/4823","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=4823"}],"version-history":[{"count":8,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/4823\/revisions"}],"predecessor-version":[{"id":5320,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=\/wp\/v2\/posts\/4823\/revisions\/5320"}],"wp:attachment":[{"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4823"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4823"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rustybolt.info\/wordpress\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4823"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}