Notes payable consisted of the following as of the dates presented (in thousands):

	June 30, 2019 (unaudited)			December 31, 2018
8% secured promissory notes due August 2018	$	—		$	250
0% convertible notes due January 2021		275			275
4% promissory note due April 2021		704			847
0% promissory note due April 2019		—			418
8% promissory note due March 2021		2,107			2,107
12% convertible notes due October 2019		4,000			4,000
12% convertible notes due November 2019		1,220			1,220
12% convertible notes due May 2020		3,500			—
Total face amount		11,806			9,117
Unamortized discount		(1,561	)		(1,780	)
Total carrying value		10,245			7,337
Amount classified as current		7,503			4,464
Amount classified as long-term	$	2,742		$	2,873

In August 2017, the Company issued a $250,000 secured promissory note with a 1-year term bearing interest at 8%. The $250,000 note was not paid upon maturity, and went into default. Upon agreement with the holder, the note was paid in full, including additional default interest on February 14, 2019.

In January 2018, the Company issued an aggregate of $1,000,000 of 3-year 0% convertible notes and warrants. The notes are convertible into an aggregate of 570,287 shares of common stock of the Company and the warrants are exercisable for 570,287 shares of common stock of the Company at an exercise price of $9.15 per share. The Company valued the debt and the warrants in accordance with ASC 470-20-25-2 using a binomial option pricing model for the warrants, and the conversion feature, which was determined to be a Beneficial Conversion Feature, was recorded at fair value based on the difference between the closing market price of the Company’s stock on the date of the transaction and the implied conversion price in the fair value of the debt. The valuation assumed a 105% volatility rate of the Company’s common stock, a risk-free interest rate of 2.20% and a credit spread of 7.70%. The warrants were assigned a value of $127,000 and the conversion feature was assigned a value of $144,000. The remaining value of $729,000 was assigned to the debt. The aggregate discount of $271,000 is being amortized to interest expense over the 3-year life of the notes on a straight-line basis. In connection with the debt exchange on August 15, 2018, holders of an aggregate principal amount of $725,000 of the notes converted their notes to common stock, leaving a principal balance of $275,000 outstanding. The unamortized discount related to the converted notes amounted to $163,000, which amount was recorded as accretion expense in the period. Accretion of the discount for the three and six months ended June 30, 2019 amounted to $6,000 and $12,000, respectively.

In April 2018, the Company issued a $1,000,000 installment note bearing interest at 4.02% per annum due April 30, 2021. The principal balance outstanding on the note at June 30, 2019 was $704,000. The note specifies varying monthly payments of principal and interest with annual principal payments of $305,000, $359,000 and $183,000 in 2019 through 2021.

In August 2018, in connection with the Unitron acquisition, the Company assumed the remaining balance of $868,000 on 0% promissory notes with $450,000 paid in October 2018 and $418,000 paid in April 2019.

In September 2018, the Company entered into a Note Consolidation Agreement with a lender in which 12 promissory notes and associated accrued interest were consolidated into single unsecured 8% promissory note in the principal amount of $2,107,000. The note is due in a lump sum on March 31, 2021. Interest compounds annually. Because the present value of the cash flows under the terms of the new debt instrument was less than 10% different from the present value of the aggregate remaining cash flows under the terms of the original instruments, the debt instruments were not considered to be substantially different and the transaction was not considered a debt extinguishment.

In October 2018, the Company issued an aggregate of $4,000,000 of 1-year 12% convertible notes and warrants. The notes are convertible at the option of the holder after six months from the date of issuance into an aggregate of 941,181 shares of common stock of the Company and the warrants are exercisable for 470,592 shares of common stock of the Company at an exercise price of $4.25 per share. The Company valued the debt and the warrants in accordance with ASC 470-20-25-2 using a binomial option pricing model for the warrants, and the conversion feature, which was determined to be a Beneficial Conversion Feature, was recorded at fair value based on the difference between the closing market price of the Company’s stock on the date of the transaction and the implied conversion price in the fair value of the debt. The valuation assumed a 90% volatility rate of the Company’s common stock, a 25% discount on the value of the underlying stock due to trading restrictions, and a risk-free interest rate of 2.47%. The warrants were assigned a value of $769,000 and the conversion feature was assigned a value of $1,173,000. The remaining value of $2,058,000 was assigned to the debt. The aggregate discount of $1,942,000 is being amortized to interest expense over the 1-year life of the notes on a straight-line basis, which approximates the effective interest method. Accretion of the discount for the three and six months ended June 30, 2019 amounted to $485,000 and $971,000, respectively.

In November 2018, the Company issued an aggregate of $1,220,000 of 1-year 12% convertible notes and warrants. The notes are convertible at the option of the holder after six months from the date of issuance into an aggregate of 277,274 shares of common stock of the Company and the warrants are exercisable for 138,638 shares of common stock of the Company at an exercise price of $4.40 per share. The Company valued the debt and the warrants in accordance with ASC 470-20-25-2 using a binomial option pricing model for the warrants. The conversion feature was not assigned any value as the implied conversion price in the fair value of the debt was higher than the closing market price of the Company’s stock on the date of the transaction. The valuation assumed a 90% volatility rate of the Company’s common stock, a 25% discount on the value of the underlying stock due to trading restrictions, and a risk-free interest rate of 2.52%. The warrants were assigned a value of $118,000 and the remaining value of $1,102,000 was assigned to the debt. The discount of $118,000 is being amortized to interest expense over the 1‑year life of the notes on a straight-line basis. Accretion of the discount for the three and six months ended June 30, 2019 amounted to $29,000 and $58,000, respectively.

In May 2019, the Company issued an aggregate of $3,500,000 of 1-year 12% convertible notes and warrants. The notes are convertible at the option of the holder after six months from the date of issuance into an aggregate of 1,258,996 shares of common stock of the Company and the warrants are exercisable for 629,500 shares of common stock of the Company at an exercise price of $2.72 per share. The Company valued the debt and the warrants in accordance with ASC 470-20-25-2 using a binomial option pricing model for the warrants, and the conversion feature, which was determined to be a Beneficial Conversion Feature, was recorded at fair value based on the difference between the closing market price of the Company’s stock on the date of the transaction and the implied conversion price in the fair value of the debt. The valuation assumed a 105% volatility rate of the Company’s common stock, a 30% discount on the value of the underlying stock due to trading restrictions, and a risk-free interest rate of 2.33%. The warrants were assigned a value of $507,000 and the conversion feature was assigned a value of $243,000. In addition, the Company incurred fundraising costs of $190,000, which were recorded as an additional discount. The remaining value of $2,750,000 was assigned to the debt. The aggregate discount of $940,000 is being amortized to interest expense over the 1-year life of the notes on a straight-line basis, which approximates the effective interest method. Accretion of the discount for the three and six months ended June 30, 2019 amounted to $117,000.

Interest expense for notes payable for the three and six months ended June 30, 2019 amounted to $900,000 and $1,628,000, respectively. Interest expense for notes payable for the three and six months ended June 30, 2018 amounted to $325,000 and $572,000, respectively.